Fill in README stubs

[hexabits]

I've added CONTRIBUTORS and TROUBLESHOOTING readmes and they will eventually need to be filled out.

@neomonkeus if you could provide a list of current and past contributors, I can format it and get it committed.

Just going by the commit graphs:

@amorilia
@Alphax
@mharj
@PacificMorrowind
@darkyellow
@NicocoiN
@infectedsoundsystem
@bobtedbob

And others have probably contributed from before GitHub.

For TROUBLESHOOTING, it will mostly be taken from the previous readme for now, and deal with improving it later. The previous readme had this:

* Do not install nifskope whilst blender is running.

* Be warned that collada export is still very experimental. It may, or
  may not, work for you.

* On some graphic cards textures show up black or distorted. If this happens,
  uncheck "Shaders" in NifSkope's settings.

* Models with multiple texture maps may appear to be missing some textures,
  but they will become visible when some texture maps are disabled. This is
  due to the limited number of texture units in graphics hardware.

* If one of NifSkope's windows disappears due to moving it to a location
  outside of the desktop view, close NifSkope and remove the NifSkope settings
  by deleting the registry key(s)

    HKEY_USERS\S-1-5-***\Software\NifTools\NifSkope-***

  Launch NifSkope again, and all windows will appear at their default
  position.

* If NifSkope does not start at all, check your firewall. NifSkope uses the
  network (locally, nothing is sent to the outside world) to communicate with
  itself.

[neomonkeus]

Additions

• NifSkope is a "Best Effort" Renderer, how it displays in the viewport does not reflect what the object may look like in-game.
• Ensure that texture paths are setup initially, Render -> Settings -> Rendering

Yep I can check back through the history.