Ignore dynamically generated doc/tags

I'm ignoring the dynamically generated ``doc/tags`` file to avoid having my submodule dirty all the time.

![git repository](https://dl.dropboxusercontent.com/u/1833867/opensource/Screen%20Shot%202013-06-24%20at%2012.52.04%20AM.png)
![dirty submodule](https://dl.dropboxusercontent.com/u/1833867/opensource/Screen%20Shot%202013-06-24%20at%2012.52.13%20AM.png)