Release Notes: repeat the GCM Core and default branch name notes

Since we're releasing v2.29.1 _so_ close after v2.29.0, let's repeat
those notes so that they are hopefully not missed by users who did not
yet upgrade to v2.29.0 and therefore skip that version.

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

Author: dscho

ReleaseNotes.md

@@ -37,6 +37,10 @@ This package contains software from a number of other projects including Bash, z
 
 ## Changes since Git for Windows v2.29.0 (October 19th 2020)
 
+Important note: v2.29.0 and v2.29.1 upgrade 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.
+
+Also, as of v2.29.0, the option to override the branch name used by `git init` for the initial branch is [featured prominently](https://github.com/git-for-windows/build-extra/pull/307) in the installer.
+
 ### New Features
 
 * The MSYS2 runtime [now optionally supports creating Cygwin-style symbolic links](https://github.com/msys2/msys2-runtime/pull/16) (via setting the environment variable `MSYS=winsymlinks:sysfile`).