docs(contributing): document one-time git hook activation

CONTRIBUTING.md

@@ -4,6 +4,21 @@
 ## Workflow guidelines
 [Wiki Page](https://git.datacontroller.io/dc/dc/wiki/Git-Workflow)
 
+## Git hooks
+
+The repo ships its own git hooks in [`.git-hooks/`](./.git-hooks):
+
+- `pre-commit` - scans staged changes for secrets with gitleaks, blocks commits that would add more than 2MB of new blobs, and runs the prettier check (auto-fixing on failure)
+- `commit-msg` - verifies the commit message follows the [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/#summary) standard
+
+The repo `.npmrc` sets `ignore-scripts=true`, so the `prepare` and `postinstall` lifecycle scripts that would normally set `core.hooksPath` during `npm i` never run. After cloning (or if your commits are not being checked), activate the hooks with a one-time command, run from the repo root:
+
+```bash
+git config core.hooksPath ./.git-hooks
+```
+
+The pre-commit hook requires the gitleaks binary provided by the root `@nogoo9/gitleaks` devDependency, so make sure `npm i` has run in the repo root before your first commit.
+
 ## Dependencies that requires licences
 
 [SheetJS Pro Version](https://www.npmjs.com/package/sheetjs)