chore: add contributing docs (#549)

CONTRIBUTING.md

@@ -0,0 +1,29 @@
+# Contributing to near-sdk-rs
+
+Thank you for your interest in contributing to NEAR's Rust SDK! We appreciate any type of contribution.
+
+If you have any questions about contributing, or about the project in general, please ask in our [rust-sdk Discord channel](https://discord.gg/cKRZCqD2b2).
+
+## Code of Conduct
+
+We have an open and welcoming environment, please review our [code of conduct](CODE_OF_CONDUCT.md).
+
+## Development
+
+### Commits
+
+Please use descriptive PR titles. We loosely follow the [conventional commits](https://www.conventionalcommits.org/en/v1.0.0/) style, but this is not a requirement to follow exactly. PRs will be addressed more quickly if it is clear what the intention is.
+
+### Before opening a PR
+
+Ensure the following are satisfied before opening a PR:
+- Code is formatted with `rustfmt` by running `cargo fmt`
+- Run `clippy`
+  - The exact command run by the CI is `cargo clippy --tests -- -Dclippy::all`
+- Run tests with `cargo test`
+- Rebuild examples with docker if any breaking changes with `./examples/build_all_docker.sh`
+  - We track changes in the example wasm blobs to make sure no unwanted code bloat is added with changes
+- Test all examples with `./examples/test_all.sh`
+  - This must be done after the previous step
+- Ensure any new functionality is adequately tested
+- If any new public types or functions are added, ensure they have appropriate [rustdoc](https://doc.rust-lang.org/rustdoc/what-is-rustdoc.html) documentation

README.md

@@ -28,6 +28,8 @@
       <a href="https://github.com/near/near-sdk-rs#building-rust-contract">Building Rust Contract</a>
       <span> | </span>
       <a href="https://docs.rs/near-sdk">Reference Documentation</a>
+      <span> | </span>
+      <a href="https://github.com/near/near-sdk-rs#contributing">Contributing</a>
     </h3>
 </div>
 
@@ -221,3 +223,7 @@ different on different machines. To be able to compile the binary in a reproduci
 that allows to compile the binary.
 
 **Use [contract-builder](https://github.com/near/near-sdk-rs/tree/master/contact-builder)**
+
+## Contributing
+
+If you are interested in contributing, please look at the [contributing guidelines](CONTRIBUTING.md).