Merge branch 'update-contributing-doc' into 'dev'

Update CONTRIBUTING doc

Closes #387

See merge request memri/pod!211

CONTRIBUTING.md

 # CONTRIBUTING 
 
-Thanks for checking, make sure to have a look at our [contributing policy](https://gitlab.memri.io/memri/constitution/blob/dev/Contributing%20Policy.md)
-Or send us a message via our [Slack](https://app.slack.com/client/TSSDHE1JN/CT4PAP7FE)
+We are excited that you are looking to contribute to memri!
+The current document is not meant as a comprehensive contribution guide,
+but some basic guidelines to keep us aligned:
+
+* It might be a good idea to reach out to us on our
+[Discord](https://discord.com/invite/BcRfajJk4k).
+You can see which issues are/aren't being worked on by other people
+in the community at the moment, and you can ask preliminary
+feedback on a feature you're thinking to implement.
+
+* We use Continuous Integration (CI) in the project.
+Your Merge Requests will likely be checked by our system,
+you have to make sure your contribution passes all tests.
+More on those tests below as well, so that you could also understand and check them locally.
+
+* We use [rustfmt](https://github.com/rust-lang/rustfmt) in the project.
+The settings are the default for Rust (100 characters line length, etc)
+
+* We use [clippy](https://github.com/rust-lang/rust-clippy) to ensure code quality in the project.
+You can run it locally by `cargo clippy --all-targets --all-features -- -D warnings`.
+
+* We make security audit of the libraries for each commit being made,
+[cargo audit](https://github.com/RustSec/cargo-audit).
+
+* We ask to [write test](https://doc.rust-lang.org/rust-by-example/testing/unit_testing.html)
+for all non-straightforward code in the project codebase.
+We might be OK with Merge Requests without tests for declarative code like initialization
+and default values, but anything involving loops/conditions is generally expected to be tested.
+
+* We do code reviews. All Merge Requests will be reviewed by somebody in our team.
+You can jump in and provide your own feedback for other Merge Requests if you want as well.
+
+* Aim to have one MR per problem, and use branches enable that.
+If you are solving a new problem before the current MR is closed, create a new MR which does not
+include the changes it the first MR. If you get a request for changes for a MR, don’t submit
+a new MR but proceed within the existing MR.
+
+    Only create MR’s that change functionality. Don’t combine them with style changes, like whitespaces, symbol naming, etc.
+
+
+We'll be happy to see your contributions on our GitLab server and any feedback
+on our [Discord](https://discord.com/invite/BcRfajJk4k)!