Release Notes: add a blurb talking about the forced upgrade to GCM Core

We will be upgrading our users from using Git Credential Manager for
Windows ("GCM(W)") to Git Credential Manager Core ("GCM Core"). This is
necessitated by GitHub's announcement to sunset password-based HTTP
authentication: GCM(W) only supports that authentication, while GCM Core
knows how to use OAuth-based authentication.

So let's provide some rationale for that.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

ReleaseNotes.md

@@ -37,6 +37,8 @@ This package contains software from a number of other projects including Bash, z
 
 ## Changes since Git for Windows v2.28.0 (July 28th 2020)
 
+This version upgrades existing users of [Git Credential Manager for Windows](https://github.com/microsoft/Git-Credential-Manager-for-Windows/) (which was just deprecated) to [Git Credential Manager Core](https://github.com/microsoft/Git-Credential-Manager-Core) ("GCM Core", which is the designated successor of the former). This is necessary because [GitHub deprecated password-based authentication](https://github.blog/changelog/2019-08-08-password-based-http-basic-authentication-deprecation-and-removal/) and intends to remove support for it soon, and GCM Core is prepared for this change.
+
 ### New Features
 
 * Comes with [cURL v7.72.0](https://curl.haxx.se/changes.html#7_72_0).