Improvements to Empirical contribution infrastructure

[emilydolson]

This pull request:

• Moves LICENSE.md to root, which is where people usually look for it.
• Adds a CONTRIBUTING.md file with instructions for how to contribute

[codecov]

Codecov Report

Merging #356 (5b9d3ef) into master (2aa1833) will increase coverage by 0.01%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #356      +/-   ##
==========================================
+ Coverage   79.02%   79.04%   +0.01%     
==========================================
  Files         306      306              
  Lines       34972    34971       -1     
==========================================
+ Hits        27637    27643       +6     
+ Misses       7335     7328       -7     

Impacted Files	Coverage Δ
include/emp/bits/BitVector.hpp	72.41% <0.00%> (ø)
include/emp/math/Random.hpp	69.19% <100.00%> (+0.56%)	⬆️

... and 3 files with indirect coverage changes

[amlalejini]

Double check that license includes all 3rd party software licenses.

[amlalejini]

Double check that we include everyone that has contributed to Empirical

[emilydolson]

@mmore500 and/or @amlalejini if one of you has a chance to look at it, this is another PR that should probably get merged before JOSS submission so that the reviewers (and users!) can easily find license and contributing information

This technically depends on #483, which is a super tiny fix that just needs someone to peek at it before it gets merged

[mmore500]

Positive changes. Not sure why this was sitting around in limbo for 3 years. ¯_(ツ)_/¯

[mmore500]

Looks great. Can we just merge this instead of #483 and close that PR?

[emilydolson]

Yeah, good call! Thanks for reviewing!

[emilydolson]

Wow, looks like GitHub auto-detects situations like this and automatically counted that PR as merged when I merged this one! Cool!