Update release.md to freeze master (#726)

* Update release.md

Author: jeremyliweishih

docs/source/changelog.rst

@@ -7,6 +7,7 @@ Changelog
     * Fixes
     * Changes
     * Documentation Changes
+        * Add instructions to freeze `master` on `release.md` :pr:`726`
     * Testing Changes
 
 

release.md

@@ -7,6 +7,7 @@ Before starting the release process, verify the following:
 * The [Read the Docs build](https://readthedocs.com/projects/feature-labs-inc-evalml/builds/) for "latest" is marked as passed.
 * The [public documentation for the "latest" branch](https://evalml.featurelabs.com/en/latest/) looks correct, and the [changelog](https://evalml.featurelabs.com/en/latest/changelog.html) includes the last change which was made on master.
 * The [performance tests](https://github.com/FeatureLabs/evalml-performance-tests) have passed on latest master, and the team has reviewed the results.
+* Communicate to the team that `master` is frozen until release PR is merged
 
 ## 1. Create release PR to update version and changelog
 A release PR should have the version number as the title (i.e. "vX.X.X") and the changelog updates as the PR body text. The PR should be based off master.
@@ -19,6 +20,7 @@ Make the following changes in the release PR:
 * Move changelog items from `Future Releases` into the correct version number.
 * Confirm that all release items are in the changelog under the correct header, and that no extra items are listed.
 
+
 An example can be found here: https://github.com/FeatureLabs/evalml/pull/163
 
 **Note**: get the PR reviewed and approved before merging. Also, verify again that all tests are currently green on master, that all checkin tests are passing, that the Read the Docs build for the release PR branch has passed and that the resulting docs contain the expected changelog. After merging, verify Read the Docs "latest" is correct.