Add developer docs for using SAML without a server (#6032)

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+# How to test SAML as a developer without a server
+
+https://capriza.github.io/samling/samling.html (https://github.com/capriza/samling) is a great
+resource for being able to tinker with the SAML options within Synapse without needing to
+deploy and configure a complicated software stack.
+
+To make Synapse (and therefore Riot) use it:
+
+1. Use the samling.html URL above or deploy your own and visit the IdP Metadata tab.
+2. Copy the XML to your clipboard.
+3. On your Synapse server, create a new file `samling.xml` next to your `homeserver.yaml` with
+   the XML from step 2 as the contents.
+4. Edit your `homeserver.yaml` to include:
+   ```yaml
+   saml2_config:
+     sp_config:
+       allow_unknown_attributes: true  # Works around a bug with AVA Hashes: https://github.com/IdentityPython/pysaml2/issues/388
+       metadata:
+         local: ["samling.xml"]   
+   ```
+5. Run `apt-get install xmlsec1` and `pip install --upgrade --force 'pysaml2>=4.5.0'` to ensure
+   the dependencies are installed and ready to go.
+6. Restart Synapse.
+
+Then in Riot:
+
+1. Visit the login page with a Riot pointing at your homeserver.
+2. Click the Single Sign-On button.
+3. On the samling page, enter a Name Identifier and add a SAML Attribute for `uid=your_localpart`.
+   The response must also be signed.
+4. Click "Next".
+5. Click "Post Response" (change nothing).
+6. You should be logged in.
+
+If you try and repeat this process, you may be automatically logged in using the information you
+gave previously. To fix this, open your developer console (`F12` or `Ctrl+Shift+I`) while on the
+samling page and clear the site data. In Chrome, this will be a button on the Application tab.