Added description of relatedness statistic to documentation.

git-svn-id: https://svn.code.sf.net/p/vcftools/code@915 19d466ce-640d-45be-954f-fad4d9839663
Added a second relatedness option (--relatedness2) to the branch.

git-svn-id: https://svn.code.sf.net/p/vcftools/code@916 19d466ce-640d-45be-954f-fad4d9839663
Merge remote-tracking branch 'upstream'