git submodule instructions are needed in the documentation

[nahumj]

As I have seen this issue arise multiple times, I think we need to have documentation explicitly say to do these steps:

git submodule init
git submodule update

I'm raising the issue instead of a pull request because I'm uncertain of the apporpiate place for this information. The README and the wiki seem like good places, and maybe a check in the build_avida with a more informative error message than CMAKE's.

[nahumj]

As Anya and Emily pointed out to me, there are indeed instuctions for doing so in the "Getting Started" page (https://github.com/devosoft/avida/wiki/Development-%7C-Getting-Started). I'll edit the README so that persons not reading the wiki won't be entirely in the dark.