Steps to run daily workflow in forked repo (#3168)

I think a lot of new contributors are not aware how they can run the
extended list of tests in their forked repo branch with other test
arguments.

This change could be helpful to some people.

---------

Signed-off-by: Sarthak Aggarwal <sarthagg@amazon.com>

CONTRIBUTING.md

@@ -76,7 +76,7 @@ If you're contributing code to the Valkey project in any other form, including
 sending a code fragment or patch via private email or public discussion groups,
 you need to ensure that the contribution is in accordance with the DCO.
 
-# How to provide a patch or a new feature
+## How to provide a patch or a new feature
 
 1. If it is a major feature or a semantical change, please don't start coding
 straight away: if your feature is not a conceptual fit you'll lose a lot of
@@ -108,4 +108,24 @@ which includes documentation about various best practices for writing Valkey cod
 To link a pull request to an existing issue, please write "Fixes #xyz" somewhere
 in the pull request description, where xyz is the issue number.
 
+## Running the daily workflow on demand for your branch
+
+Use [`.github/workflows/daily.yml`](.github/workflows/daily.yml) with
+`workflow_dispatch` to run daily tests manually on any branch in your fork.
+
+1. Open your fork on GitHub and go to **Actions** -> **Daily**.
+2. Click **Run workflow**.
+3. In the **Branch** dropdown, select the branch that contains the workflow file you want to use.
+4. In the input fields, set:
+   * `use_repo` to your fork (for example, `your-user/valkey`)
+   * `use_git_ref` to your branch name (or a specific commit SHA)
+5. Optionally set `skipjobs`, `skiptests`, `test_args`, and `cluster_test_args`.
+6. Click **Run workflow**.
+
+Notes:
+* To run the full matrix, set `skipjobs` and `skiptests` to `none`.
+  Do not leave them empty, since the workflow input defaults may be applied.
+* The scheduled part of this workflow is gated to `valkey-io/valkey`, but manual
+  `workflow_dispatch` runs work for forks.
+
 Thanks!