add contributing instructions

CONTRIBUTING.md

@@ -0,0 +1,8 @@
+Pull requests & issues welcome!
+
+If you spot an error in the course material, you can file an issue or
+a pull request pointing out the error.
+
+Unfortunately, I'm not able to directly merge pull requests into this
+repository, as the contents of this repository is generated from
+another, secret, repository.

README.md

@@ -35,3 +35,9 @@ of the set you are working on.
 
 
 See [the material](part1.html#working-on-the-exercises) for more info.
+
+## Reporting errors
+
+If you notice an error in these materials, you can report it via
+- an issue or pull request in this repository (see [CONTRIBUTING.md](CONTRIBUTING.md))
+- the course [channel on Telegram](https://t.me/haskell_mooc_fi)