Add Service Discovery (/.well-known/matrix/client) support

docs/configuring-well-known.md

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+# Configuring service discovery via .well-known
+
+
+## Introduction
+
+Service discovery lets various client programs which support it, to receive a full user id (e.g. `@username:example.com`) and determine where the Matrix server is automatically (e.g. `https://matrix.example.com`).
+
+This lets your users easily connect to your Matrix server without having to customize connection URLs.
+
+As [per the specification](https://matrix.org/docs/spec/client_server/r0.4.0.html#server-discovery) Matrix does service discovery using a `/.well-known/matrix/client` file hosted on the base domain (e.g. `example.com`).
+
+However, this playbook installs your Matrix server on another domain (e.g. `matrix.example.com`) and not on the base domain (e.g. `example.com`), so it takes a little extra manual effort to set up the file.
+
+
+## Prerequisites
+
+To implement service discovery, your base domain's server (e.g. `example.com`) needs to support HTTPS.
+
+
+## Setting it up
+
+To make things easy for you to set up, this playbook generates and hosts the well-known file on the Matrix domain's server (e.g. `https://matrix.example.com/.well-known/matrix/client`), even though this is the wrong place to host it.
+
+You have 2 options when it comes to installing the file on the base domain's server:
+
+1) (Option 1): **Copying the file manually** to your base domain's server
+
+All it takes is copying the `/.well-known/matrix/client` from the Matrix server (e.g. `matrix.example.com`) to your base domain's server (`example.com`).
+
+This is easy to do and possibly your only choice if you can only host static files from the base domain's server.
+It is, however, a little fragile, as future updates performed by this playbook may regenerate the well-known file and you may need to notice that and copy it again.
+
+2) (Option 2): **Setting up reverse-proxying** of the well-known file from the base domain's server to the Matrix server.
+
+This option is less fragile and generally better.
+
+On the base domain's server (e.g. `example.com`), you can set up reverse-proxying, so that any access for the `/.well-known/matrix` location prefix is forwarded to the Matrix domain's server (e.g. `matrix.example.com`).
+
+**For nginx**, it would be something like this:
+
+```nginx
+# This is your HTTPS-enabled server for DOMAIN.
+server {
+	server_name DOMAIN;
+
+	location /.well-known/matrix {
+		proxy_pass https://matrix.DOMAIN/.well-known/matrix;
+		proxy_set_header X-Forwarded-For $remote_addr;
+	}
+
+	# other configuration
+}
+```
+
+**For Apache**, it would be something like this:
+
+```apache
+<VirtualHost *:443>
+	ServerName DOMAIN
+
+	SSLProxyEngine on
+	<Location /.well-known/matrix>
+		ProxyPass "https://matrix.DOMAIN/.well-known/matrix"
+	</Location>
+
+	# other configuration
+</VirtualHost>
+```
+
+Make sure to:
+
+- **replace `DOMAIN`** in the server configuration with your actual domain name
+- and: to **do this for the HTTPS-enabled server block**, as that's where Matrix expects the file to be
+
+
+## Confirming it works
+
+No matter which method you've used to set up the well-known file, if you've done it correctly you should be able to see a JSON file at a URL like this: `https://matrix.<domain>/.well-known/matrix/client`.