Add Contributing.md or other resource for potential contributors

[jajik]

At the moment, there are several checks on commits that are not documented anywhere (?). It would be nice to list these requirements to prevent unnecessary confusion & CI failures.

Additionally, such a document would be a good place to mention code style guidelines (if any), etc.

[Noltari]

It might not be documented in this repo, but the same rules for general OpenWrt contributions still apply here:
https://openwrt.org/submitting-patches

[jajik]

Yes, this

omcproxy/.github/workflows/formal.yml

Line 12 in 11e45f6

uses: openwrt/actions-shared-workflows/.github/workflows/formal.yml@main

hints some general rules as well, but I wasn't aware of these tbh.

May I suggest adding a little section into the README.md? Something like

## Contributing

omcproxy is part of the OpenWrt Project, and as such, its contribution guidelines apply. You can find them [here](https://openwrt.org/submitting-patches).