docs: Community health files

Author: obalunenko

github/CODE_OF_CONDUCT.md renamed to .github/CODE_OF_CONDUCT.md

@@ -1,4 +1,3 @@
-# Contributor Covenant Code of Conduct
 
 ## Our Pledge
 

.github/CONTRIBUTING.md

@@ -0,0 +1,66 @@
+# Contributing
+
+By participating to this project, you agree to abide [code of conduct](https://github.com/obalunenko/advent-of-code/blob/master/.github/CODE_OF_CONDUCT.md).
+
+Please note that I'll not accept any PR with new puzzle solutions - I don't like spoilers.
+
+But if you have any idea how to improve the existing codebase - you are welcome to create a PR.
+
+## Setup your machine
+
+`advent-of-code` is written in [Go](https://golang.org/).
+
+Prerequisites:
+
+- [Go 1.17+](https://golang.org/doc/install)
+- make
+
+Clone `advent-of-code` anywhere:
+
+```sh
+git clone git@github.com:obalunenko/advent-of-code.git
+```
+
+`cd` into the directory and install the dependencies:
+
+```sh
+make install-tools
+```
+
+A good way of making sure everything is all right is running the test suite:
+
+```sh
+make test
+```
+
+## Test your change
+
+You can create a branch for your changes and try to build from the source as you go:
+
+```sh
+make build
+```
+
+When you are satisfied with the changes, we suggest you run:
+
+```sh
+make test && make test-regression
+```
+
+Before you commit the changes, we also suggest you run:
+
+```sh
+make format-project
+```
+
+## Create a commit
+
+Commit messages should be well formatted, and to make that "standardized", we
+are using Conventional Commits.
+
+You can follow the documentation on
+[their website](https://www.conventionalcommits.org).
+
+## Submit a pull request
+
+Push your branch to your `advent-of-code` fork and open a pull request against the main branch.