some docs for contributors - fix #20

CONTRIBUTING.md

+# How to contribute
+
+This documentation is built with Hugo plus Hyas.
+
+To build locally:
+
+```bash
+npm install
+npm run build
+```
+
+To contribute some changes:
+
+* work on a branch
+* make your changes (usually inside `content/`)
+* check how your changes look by running `npm run dev` and pointing
+  your browser at the web servers it creates
+* once you're done, `npm run lint` to check your formatting
+* commit, push, open a MR on the GitLab
+
+A word about the `weight:` attribute of pages: it's used to sort pages
+for the "next" / "prev" links at the bottom. If you add a new page,
+make sure it's slotted in a sensible place in that order.

README.md

 # Documentation
 
-TBD
+This is the documentation repository for the Sharkey project, see
+https://activitypub.software/TransFem-org/Sharkey/
+
+It's published at https://docs.joinsharkey.org/