Add CONTRIBUTING

Author: oprypin

CONTRIBUTING.md

@@ -0,0 +1,27 @@
+# Development guide
+
+## Trying it out locally
+
+### Example
+
+```bash
+tmp=$(mktemp -d) && touch "$tmp"/{path,env}
+export RUNNER_TEMP="$tmp" GITHUB_PATH="$tmp/path" GITHUB_ENV="$tmp/env"
+env PATH="$(pwd)/.tools:$PATH" INPUT_CRYSTAL=nightly INPUT_SHARDS=true INPUT_TOKEN="$personal_access_token" INSTALL_CRYSTAL_PLATFORM=win32 node index.js
+```
+
+## Pre-commit
+
+Make sure that `npm test` passes. A pre-commit hook is available: `git config core.hooksPath .tools/hooks`.
+
+## CI
+
+Continuous testing runs on GitHub Actions. Make sure that _.github/workflows/main.yml_ and _release.yml_ stay in sync, other than the differences that they already have.
+
+## Cutting a release
+
+Switch to _master_ branch, in a clean state. Run `.tools/release.sh x.y.z` (an actual next version number). You will be switched to the _v1_ branch, _node_modules_ will be populated, and a tag will be created. The final command to push all this will be only printed. Run it. After this, a release also needs to be published on GitHub Marketplace.
+
+### _node_modules_
+
+The _node_modules_ directory must not be present in the _master_ branch. _package-lock.json_ will be present, though.