Bug 1497575: [staging-release] Add some docs for `mach try release`; r=Callek

Differential Revision: https://phabricator.services.mozilla.com/D8768

--HG--
extra : moz-landing-system : lando

tools/tryselect/docs/selectors/index.rst

@@ -11,6 +11,7 @@ These are the currently implemented try selectors:
   some tasks automatically (like lint and python unittest tasks). Further tasks
   can be chosen with treeherder's ``Add New Jobs`` feature.
 * :doc:`syntax <syntax>`: Select tasks using classic try syntax.
+* :doc:`release <release>`: Prepare a tree for doing a staging release.
 
 You can run them with:
 
@@ -32,3 +33,4 @@ See selector specific options by running:
   Fuzzy <fuzzy>
   Empty <empty>
   Syntax <syntax>
+  Release <release>

tools/tryselect/docs/selectors/release.rst

@@ -0,0 +1,27 @@
+Release Selector
+================
+
+This command configures the tree in preparation for doing a staging release,
+and pushes the result to try. The changes that that are made include:
+
+- Updating the version number.
+- Applying the migrations that are done as part of merge day.
+- Disabling repacking most locales. (This can be disabled by passing ``--no-limit-locales``).
+
+For staging a beta release, run the following (with an appropriate version number):
+
+.. code-block:: shell
+
+    $ mach try release --version 64.0b5 --migration central-to-beta
+
+For staging a final release (rc or patch), run the following (with an appropriate version number)
+
+.. code-block:: shell
+
+    $ mach try release --version 64.0 --migration central-to-beta --migration beta-to-release
+
+Once the decision task is on the push is complete, you can start the release
+through `staging ship-it instance <https://shipit.staging.mozilla-releng.net/new>`_\ [#shipit]_.
+
+
+.. [#shipit] This is only available to release engineering and release management (as of 2018-10-15).