chore: add Contributing.md (#204)

CONTRIBUTING.md

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+# Contributing
+
+
+Thank you for considering to contribute to this project. KYVE is an L1 based
+on Cosmos SDK and CometBFT. We mostly follow their principles and design
+architectures.
+
+## Overview
+
+- The latest state of development is on `main`.
+- `main` must always pass `make all ENV=mainnet`.
+- Releases can be found in `/release/*`.
+- Everything must be covered by tests. We have a very extensive test-suite
+  and use triple-A testing (Arrange, Act, Assert).
+
+## Creating a Pull Request
+
+- Check out the latest state from main and always keep the PR in sync with main.
+- Use [conventional commits](https://www.conventionalcommits.org/en/v1.0.0/#specification).
+- Only one feature per pull request.
+- Write an entry for the Changelog.
+- Write tests covering 100% of your modified code.
+- The command `make all ENV=mainnet` must pass. 
+
+## Coding Guidelines
+
+- Write readable and maintainable code. `Premature Optimization Is the Root of All Evil`.
+  Concentrate on clean interfaces first and only optimize for performance if it is needed.
+- The keeper directory is structured the following:
+  - `getters_*`-files only interact with the KV-Store. All methods always succeed
+    and do not return errors. This is the only place where methods are allowed to 
+    write to the KV-Store. Also, all aggregation variables are updated here.
+  - `logic_*`-files handle complex tasks and are encouraged to emit events and
+    call the getters functions. 
+  - `msg_server_*`-files are the entry point for message handling. This file
+    should be very clean to read and outsource most of the part to the logic files.
+    One should immediately understand the flow by just reading the function names
+    which are called while handling the message.
+
+## Chain Upgrade
+
+If a major chain upgrade is planned, the additional checks must be done:
+- Check if the AnteHandler (app/ante.go:L45) is still up-to-date.
+- Check ABCI++ Proposal Handler extensions
+- Carefully read the upgrade guides for CosmosSDK and IBC as well as CometBFT
+- Get in contact with block explorer maintainers and notify them about upcoming changes.
+- Test if Ledger is still working
+- Update the [docs](https://docs.kyve.network).
+
+## Legal
+
+You agree that your contribution is licenced under the MIT Licence and all
+ownership is handed over the authors named in [LICENSE](https://github.com/KYVENetwork/chain/blob/main/LICENSE).