Add documentation for field trials

Bug: webrtc:14154
Change-Id: I8866933046bb3c673b55fc8edf5fdbff11b60fad
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/266481
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Emil Lundmark <lndmrk@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#37309}

g3doc/field-trials.md

@@ -0,0 +1,45 @@
+# Field trials
+
+<?% config.freshness.owner = 'lndmrk' %?>
+<?% config.freshness.reviewed = '2022-06-23' %?>
+
+WebRTC provides some means to alter its default behavior during run-time,
+colloquially known as *field trials*. This is foremost used for A/B testing new
+features and are related to
+[Chromium field trials](https://chromium.googlesource.com/chromium/src/+/main/testing/variations/README.md)
+to facilitate interoperability.
+
+A field trial consist of a key-value pair of strings. By convention, the field
+trial key is prefixed with `WebRTC-` and each word is capitalized without
+spaces. Sometimes the key is further subdivided into a category, for example,
+`WebRTC-MyCategory-MyExperiment`. The field trial value is an opaque string and
+it is up to the author to define what it represents. There are
+[helper functions](https://webrtc.googlesource.com/src/+/refs/heads/main/api/field_trials_view.h)
+to use a field trial as a boolean, with the string `Enabled` representing true
+and `Disabled` representing false. You can also use
+[field trial parameters](https://webrtc.googlesource.com/src/+/refs/heads/main/rtc_base/experiments/field_trial_parser.h)
+if you wish to encode more elaborate data types.
+
+The set of field trials can be instantiated from a single string with the format
+`<key-1>/<value-1>/<key-2>/<value-2>/`. Note the final `/` at the end! In
+Chromium you can launch with the `--force-fieldtrials` flag to instantiate field
+trials this way, for example:
+
+```
+--force-fieldtrials="WebRTC-Foo/Enabled/WebRTC-Bar/Disabled/"
+```
+
+## Policy
+
+The policy for field trials is:
+
+-   A field trial should only be used to test out new code or parameters for a
+    limited time period. It should not be used for configuring persistent
+    behavior.
+-   The field trial must have an end date. The end date may be pushed back if
+    necessary, but should not be pushed back indefinitely.
+-   A field trial must be associated with a bug that
+    -   reserves the field trial key,
+    -   is open,
+    -   is assigned to an owner, and
+    -   has the end date specified.

g3doc/sitemap.md

@@ -46,3 +46,5 @@
     *   Network emulation
         *   [Implementation](/test/network/g3doc/index.md)
     *   Performance stats collection
+*   Experimentation
+    *   [Field trials](/g3doc/field-trials.md)