Move the testing guide

Author: eikendev

docs/docs/developer-guide.md

@@ -1,3 +1,34 @@
 # Developer Guide
 
 _This site is under construction._
+
+## Testing
+
+Testing is essential for delivering good and reliable software.
+PushBits uses Go's integrated test features.
+Unfortunately, writing tests is quite time consuming and therefore not every feature and every line of code is automatically tested.
+Feel free to help us improve our tests.
+
+To run tests for a single (sub)module you can simply execute the following command in the module's folder.
+
+```bash
+go test
+```
+
+To get the testing coverage for a module use the `-cover` flag.
+
+```bash
+go test -cover
+```
+
+To execute a single test use the `-run` flag.
+
+```bash
+go test -run "TestApi_getUser"
+```
+
+Running tests for all PushBits module is done like this:
+
+```bash
+make test
+```