add link with how-to for external contributions to docs on learn platform (#1513)

## Why make this change?
- change to add link with how-to for external contributions to docs on
learn platform

## What is this change?
- add link with how-to for external contributions to docs on learn
platform

## How was this tested?
-

---------

Co-authored-by: Sean Leonard <[email protected]>
Co-authored-by: Aniruddh Munde <[email protected]>

Author: 3 people

docs/readme.md

@@ -1,3 +1,5 @@
 # Documentation
 
 All documentation has now been moved [here](https://learn.microsoft.com/azure/data-api-builder/)
+
+The process for updating Data API builder documentation on Learn is available [here](https://learn.microsoft.com/contribute/)