Updated so that the headings display properly

Previously headings would just display with two ** on either side of the heading.

The proper markdown is to use '-' underneath the heading.

CONTRIBUTING.md

@@ -1,5 +1,5 @@
-** How to report a bug **
-
+How to report a bug
+-------------------
 - Use the search function to see if someone else has already reported the bug. If an issue has already been opened, see if you can add any useful information to it.
 - Otherwise, create a new issue on the "issues" page
 - Describe the problem as detailed as possible.
@@ -8,20 +8,20 @@
 - If possible, add instructions on how to reproduce the bug.
 - If possible, add a logfile to your post. This is especially useful if the bug makes the application crash. You can create logfiles with an app like `aLogcat`. Just launch `alogcat`, then start AntennaPod and reproduce the bug. aLogcat should now display information about the bug when you start it.
 
-** How to submit a feature request **
-
+How to submit a feature request
+-------------------------------
 - Make sure you are using the latest version of AntennaPod. Perhaps the feature you are looking for has already been implemented.
 - Use the search function to see if someone else has already submitted the same feature request. 
 - If an issue has already been opened, see if you can add anything useful to it.
 - Otherwise, create a new issue on the "issues" page  
 - Give a brief explanation about the problem that currently exists and why your requested feature solves this problem.
 
-** Translating AntennaPod **
-
+Translating AntennaPod
+----------------------
 If you would like to translate the app into another language or improve an existing translation, you can visit the [Transifex project page](https://www.transifex.com/projects/p/antennapod/). From there, you can either join a language team if it already exists or create a new language team.
 
-** Submit a pull-request **
-
+Submit a pull-request
+---------------------
 - If you want to work on a feature that has been requested or fix a bug that has been reported on the "issues" page, add a comment to it so that other people know that you are working on it.  
 - Fork the repository  
 - Almost all changes of AntennaPod are done on the `develop` branch. If a new version of AntennaPod is released, the `develop` branch is merged into `master`. As a result, the `master` branch probably doesn't contain the latest changes when you are reading this. Please make sure that you are branching from `develop`! Otherwise, there might be a lot of merge-conflicts when merging your changes into `develop` and therefore it might take longer to review your pull-request. Exceptions are changes in files like README.md, CONTRIBUTING.md and other files that are not directly related to the source code.  
@@ -29,6 +29,6 @@ If you would like to translate the app into another language or improve an exist
 - Although not every part of AntennaPod's source code is fully documented yet, it would be very nice if you could add documentation to your changes if it is a larger pull-request.  
 - If possible, add unit tests for your pull-request and make sure that they pass. Information on how to add unit tests and run them can be found here: [TODO] 
 
-** Improving this file **
-
-If you think this file needs clarification or additional information on certain topics, feel free to improve it via pull-requests or ask a question on the issues section.
+Improving this file
+-------------------
+If you think this file needs clarification or additional information on certain topics, feel free to improve it via pull-requests or ask a question on the issues section.