Add note for organization contributors about creating branches direct…

…ly (paritytech#7611)

# Description

* This PR adds a note to the CONTRIBUTING.md file to inform contributors
who are part of the organization that they do not need to fork the
repository. Instead, they can create a branch directly in the repository
to send a pull request.

## Changes

Added a note under the "What?" section in CONTRIBUTING.md to clarify
that organization contributors can create branches directly in the
repository.

docs/contributor/CONTRIBUTING.md

@@ -7,6 +7,9 @@ The `Polkadot SDK` project is an **OPENISH Open Source Project**
 Individuals making significant and valuable contributions are given commit-access to the project. Contributions are done
 via pull-requests and need to be approved by the maintainers.
 
+> **Note:** Contributors who are part of the organization do not need to fork the repository. They can create a branch
+> directly in the repository to send a pull request.
+
 ## Rules
 
 There are a few basic ground-rules for contributors (including the maintainer(s) of the project):