[doc] Update contributing

CONTRIBUTING.md

@@ -2,16 +2,16 @@ Most importantly: Many thanks for considering contributing to bumblebee-status!
 
 One thing I need to mention: This is a project I am working on in my (limited) spare time. I try very hard to answer bug tickets and review Pull Requests as quickly as possible, but it might take days, in some cases even weeks, until I get around to doing so. I want to give every contribution the attention it deserves. Really: I am not ignoring you, I'm simply slow :-)
 
-### Filing a Bug
+### Filing a bug
 If you want to file a bug, simply open an issue and describe your problem. Things that help narrow down the problem are:
-* Steps to reproduce
-* Relevant section of the i3 configuration
-* Debug logs and console output of bumblebee-status
+- Steps to reproduce
+- Relevant section of the i3 configuration
+- Debug logs and console output of bumblebee-status
 
 But even if you can't provide those, any indicator that something is not working as it should is much appreciated!
 
-### Adding a new Module or Theme
-If you want to add a new module, please have a look at [How to write a new module](https://github.com/tobi-wan-kenobi/bumblebee-status/wiki/How-to-write-a-module) and [How to write a new theme](https://github.com/tobi-wan-kenobi/bumblebee-status/wiki/How-to-write-a-theme). Then simply create a Pull Request and I will review the changes as soon as possible.
+### Adding a new module or theme
+If you want to add a new module, please have a look at [how to write a new module](doc/HOWTO_MODULE.md) and [how to write a new theme](doc/HOWTO_THEME.md). Then simply create a Pull Request and I will review the changes as soon as possible.
 If you want to do me a *big* favour, check the Travis status for any failing unit tests. Oh - and if you happen to add unit tests, that's also something I am very grateful for!
 
 Thanks for reading until here! :)