added guide for SAML settings

docs/guides/auth.md

@@ -75,3 +75,138 @@ To do this Click your profile icon --> Settings and privacy --> Mobile  --> Sele
       HMD_GITHUB_CLIENTID=3747d30eaccXXXXXXXXX
       HMD_GITHUB_CLIENTSECRET=2a8e682948eee0c580XXXXXXXXXXXXXXXXXXXXXX
       ````
+
+### SAML (OneLogin)
+1. Sign-in or sign-up for an OneLogin account. (available free trial for 2 weeks)
+2. Go to the administration page.
+3. Select the **APPS** menu and click on the **Add Apps**.
+
+![onelogin-add-app](images/auth/onelogin-add-app.png)
+
+4. Find "SAML Test Connector (SP)" for template of settings and select it.
+
+![onelogin-select-template](images/auth/onelogin-select-template.png)
+
+5. Edit display name and icons for OneLogin dashboard as you want, and click **SAVE**.
+
+![onelogin-edit-app-name](images/auth/onelogin-edit-app-name.png)
+
+6. After that other tabs will appear, click the **Configuration**, and fill out the below items, and click **SAVE**.
+    * RelayState: The base URL of your hackmd, which is issuer. (last slash is not needed)
+    * ACS (Consumer) URL Validator: The callback URL of your hackmd. (serverurl + /auth/saml/callback)
+    * ACS (Consumer) URL: same as above.
+    * Login URL: login URL(SAML requester) of your hackmd. (serverurl + /auth/saml)
+
+![onelogin-edit-sp-metadata](images/auth/onelogin-edit-sp-metadata.png)
+
+7. The registration is completed. Next, click **SSO** and copy or download the items below.
+    * X.509 Certificate: Click **View Details** and **DOWNLOAD** or copy the content of certificate ....(A)
+    * SAML 2.0 Endpoint (HTTP): Copy the URL ....(B)
+
+![onelogin-copy-idp-metadata](images/auth/onelogin-copy-idp-metadata.png)
+
+8. In your hackmd server, create IdP certificate file from (A)
+9. Add the IdP URL (B) and the Idp certificate file path to your config.json file or pass them as environment variables.
+    * config.json:
+      ````javascript
+      {
+        "production": {
+          "saml": {
+            "idpSsoUrl": "https://*******.onelogin.com/trust/saml2/http-post/sso/******",
+            "idpCert": "/path/to/idp_cert.pem"
+          }
+        }
+      }
+      ````
+    * environment variables
+      ````
+      HMD_SAML_IDPSSOURL=https://*******.onelogin.com/trust/saml2/http-post/sso/******
+      HMD_SAML_IDPCERT=/path/to/idp_cert.pem
+      ````
+10. Try sign-in with SAML from your hackmd sign-in button or OneLogin dashboard (like the screenshot below). 
+
+![onelogin-use-dashboard](images/auth/onelogin-use-dashboard.png)
+
+### SAML (Other cases)
+The basic procedure is the same as the case of OneLogin which is mentioned above. If you want to match your IdP, you can use more configurations as below.
+
+* If your IdP accepts metadata XML of the service provider to ease configuraion, use this url to download metadata XML.
+    * {{your-serverurl}}/auth/saml/metadata
+    * _Note: If not accessable from IdP, download to local once and upload to IdP._
+* Change the value of `issuer`, `identifierFormat` to match your IdP.
+  * `issuer`: A unique id to identify the application to the IdP, which is the base URL of your HackMD as default
+  * `identifierFormat`: A format of unique id to identify the user of IdP, which is the format based on email address as default. It is recommend that you use as below.
+    * urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress (default)
+    * urn:oasis:names:tc:SAML:1.1:nameid-format:unspecified
+  * config.json:
+    ````javascript
+    {
+      "production": {
+        "saml": {
+          /* omitted */
+          "issuer": "myhackmd"
+          "identifierFormat": "urn:oasis:names:tc:SAML:1.1:nameid-format:unspecified"
+        }
+      }
+    }
+    ````
+  * environment variables
+    ````
+    HMD_SAML_ISSUER=myhackmd
+    HMD_SAML_IDENTIFIERFORMAT=urn:oasis:names:tc:SAML:1.1:nameid-format:unspecified
+    ````
+
+* Change mapping of attribute names to customize the displaying user name and email address to match your IdP.
+  * `attribute`: A dictionary to map attribute names
+  * `attribute.id`: A primary key of user table for your HackMD
+  * `attribute.username`: Attribute name of displaying user name on HackMD
+  * `attribute.email`: Attribute name of email address, which will be also used for Gravatar
+    * _Note: Default value of all attributes is NameID of SAML response, which is email address if `idfentifierFormat` is default._
+  * config.json:
+    ````javascript
+    {
+      "production": {
+        "saml": {
+          /* omitted */
+          "attribute": {
+            "id": "sAMAccountName",
+            "username": "displayName",
+            "email": "mail"
+          }
+        }
+      }
+    }
+    ````
+  * environment variables
+    ````
+    HMD_SAML_ATTRIBUTE_ID=sAMAccountName
+    HMD_SAML_ATTRIBUTE_USERNAME=nickName
+    HMD_SAML_ATTRIBUTE_EMAIL=mail
+    ````
+
+* If you want to controll permission by group membership, add group attribute name and required group (allowed) or external group (not allowed).
+  * `groupAttribute`: An attribute name of group membership
+  * `requiredGroups`: Group names array for allowed access to HackMD. Use vertical bar to separate for environment variables.
+  * `externalGroups`: Group names array for not allowed access to HackMD. Use vertical bar to separate for environment variables.
+    * _Note: Evaluates `externalGroups` first_
+  * config.json:
+    ````javascript
+    {
+      "production": {
+        "saml": {
+          /* omitted */
+          "groupAttribute": "memberOf",
+          "requiredGroups": [ "hackmd-users", "board-members" ],
+          "externalGroups": [ "temporary-staff" ]
+        }
+      }
+    }
+    ````
+  * environment variables
+    ````
+    HMD_SAML_GROUPATTRIBUTE=memberOf
+    HMD_SAML_REQUIREDGROUPS=hackmd-users|board-members
+    HMD_SAML_EXTERNALGROUPS=temporary-staff
+    ````
+
+