Add deprecation warning to theia/git readme

Signed-off-by: Jonas Helming <jhelming@eclipsesource.com>

#12745

packages/git/README.md

@@ -5,7 +5,11 @@
 <img src='https://raw.githubusercontent.com/eclipse-theia/theia/master/logo/theia.svg?sanitize=true' alt='theia-ext-logo' width='100px' />
 
 <h2>ECLIPSE THEIA - GIT EXTENSION</h2>
-
+<h2>This extension is DEPRECATED!</h2>
+<h3>
+It has a functional overlap with the VS Code built-in Git extension and it does not make sense to maintain our own Git extension anymore. This will reduce maintenance efforts.
+If you are an active user of the Theia Git extension, we recommend you to evaluate switching to the VS Code built-in. If you see any issues with the switch, please raise them here.
+</h3>
 <hr />
 
 </div>