pre-commit: adding pre-commit

.pre-commit-config.yaml

@@ -0,0 +1,11 @@
+fail_fast: true
+files: ^(src|test)/
+repos:
+- repo: local
+  hooks:
+  - id: flake8
+    name: flake8
+    entry: flake8
+    language: system
+    files: ^src/
+    types: [python]

CONTRIBUTING.md

@@ -146,6 +146,16 @@ Additionally,
   - Docs are built with Sphinx, use rst syntax.
   - Follow the usual convention 1 line summary, blank line, description.
 
+### Use pre-commit
+
+The repository comes with a [pre-commit](https://pre-commit.com/) config to
+check the code with flake8 before commiting.
+
+Install the git-hook with:
+```sh
+pre-commit install
+```
+
 ## Documentation
 
 Pyrocko's documentation is built using the `Sphinx` tool. See the `docs`