Issue Tracker transition note (#2169)

CONTRIBUTING.md

@@ -39,9 +39,11 @@ For acceptable evidence, [see below](#provide-supporting-evidence).
 *   Link from demo (http://libphonenumber.appspot.com) showing error:
 ```
 
-From mid December to early January, we kindly request that you use Google's [Issue Tracker](http://issuetracker.google.com/issues/new?component=192347)
-to report issues with the library. During this period we'll turn off the "Issues" tab here
-on GitHub. We apologize for any inconvenience (e.g. accessing old report) while we test this system!
+As of March 2018, we kindly request that you use Google's [Issue Tracker](http://issuetracker.google.com/issues/new?component=192347)
+to report issues with the library. Using Issue Tracker will enable to us to
+get back to you more efficiently as we align GitHub user issues with ones
+reported at Google. Note that we transitioned active GitHub issues to Issue Tracker,
+but anything inactive for over a year must be filed again.
 
 Alternatively, if you have a particular issue with a Google product handling phone numbers (e.g. signup),
 you may also use the [Google Product Forums](https://productforums.google.com/forum).
@@ -123,9 +125,7 @@ To get your pull request merged, we need the following:
 *   A CLA signature. See below.
 *   Responsiveness to back and forth review comments, because we will do a code
     review.
-*   A link to a clear description of what the pull request intends to solve in a
-    [GitHub issue](https://github.com/googlei18n/libphonenumber/issues/new) or
-    Google's [Issue
+*   A link to a clear description of what the pull request intends to solve in [Issue
     Tracker](http://issuetracker.google.com/issues/new?component=192347).
 *   Tests that illustrate the issue, if applicable.
 *   Willingness and ability to implement the changes in C++, Java, and JS if