Add text in contribution.md

CONTRIBUTING.md

@@ -1,4 +1,10 @@
-TODO. :)
+# Contributing
 
-In the meantime, please file an issue on GitHub so we can talk about how best
-to accommodate your contribution.
+Flask-Talisman loves help from anyone in the community. You can do so in the following steps:
+
+- Fork the repository.
+- Write your contribution. Don't forget to add tests!
+- Run tests and linting locally by running `nox` in the root of the project.
+- If everything passes, submit a [pull request](https://github.com/wntrblm/flask-talisman/compare/).
+
+At this point you're waiting on us. We like to at least comment on pull requests within a few days.