[doc] Add basic HOWTO write a module

doc/HOWTO_MODULE.md

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+# Writing a bumblebee-status module
+
+## Introduction
+Adding a new module to `bumblebee-status` is straight-forward:
+
+- Add a new Python module in `modules/contrib/`. The name of the
+  module will be the name that the user needs to specify when
+  invoking `bumblebee-status` (i.e. a module called `modules/contrib/test.py`
+  will be loaded using `bumblebee-status -m test`)
+- See below for how to actually write the module
+- Test (run `bumblebee-status` in the CLI)
+- Make sure your changes don't break anything: `./coverage.sh`
+
+## Pull requests
+The project **gladly** accepts PRs for bugfixes, new functionality, new
+modules, etc.
+When you feel comfortable with what you've developed, please just open
+a PR, somebody will look at it eventually :) Thanks!
+
+## Hello world
+This example will show "hello world" in the status bar:
+
+```
+"""Short description"""
+
+import core.module
+import core.widget
+
+class Module(core.module.Module):
+    def __init__(self, config):
+        super().__init__(config, core.widget.Widget(self.full_text))
+
+    def full_text(self, widgets):
+        return "hello world"
+
+# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4
+```