Add documentation for experimental feature flags. (#10865)

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+# Implementing experimental features in Synapse
+
+It can be desirable to implement "experimental" features which are disabled by
+default and must be explicitly enabled via the Synapse configuration. This is
+applicable for features which:
+
+* Are unstable in the Matrix spec (e.g. those defined by an MSC that has not yet been merged).
+* Developers are not confident in their use by general Synapse administrators/users
+  (e.g. a feature is incomplete, buggy, performs poorly, or needs further testing).
+
+Note that this only really applies to features which are expected to be desirable
+to a broad audience. The [module infrastructure](../modules/index.md) should
+instead be investigated for non-standard features.
+
+Guarding experimental features behind configuration flags should help with some
+of the following scenarios:
+
+* Ensure that clients do not assume that unstable features exist (failing
+  gracefully if they do not).
+* Unstable features do not become de-facto standards and can be removed
+  aggressively (since only those who have opted-in will be affected).
+* Ease finding the implementation of unstable features in Synapse (for future
+  removal or stabilization).
+* Ease testing a feature (or removal of feature) due to enabling/disabling without
+  code changes. It also becomes possible to ask for wider testing, if desired.
+
+Experimental configuration flags should be disabled by default (requiring Synapse
+administrators to explicitly opt-in), although there are situations where it makes
+sense (from a product point-of-view) to enable features by default. This is
+expected and not an issue.
+
+It is not a requirement for experimental features to be behind a configuration flag,
+but one should be used if unsure.
+
+New experimental configuration flags should be added under the `experimental`
+configuration key (see the `synapse.config.experimental` file) and either explain
+(briefly) what is being enabled, or include the MSC number.