Add explanation how to run tests to README.md

README.md

+# Building
+
+1. First install `redo` and `scdoc` from the community repository
+2. Create the man pages with `redo build`
+
+# Testing
+
+1. First install `redo` and `bats` from the community repository
+1. Install `grep` from the main repository
+1. Make sure the directory containing `./apkbuild-lint` is on `$PATH`
+1. Run the tests with `redo check`