Mention Sass precision Bootstrap fix

If your buttons line-height is 1.42857 instead of 1.428571429, this is because the precision is set to 5 instead of 10.

Digging into bootstrap-sass code, it looks like it [should be automatically fixed](https://github.com/twbs/bootstrap-sass/blob/master/lib/bootstrap-sass.rb#L56) but did not work in my configuration at least.
Merge pull request #107 from thbar/patch-1

Mention Sass precision Bootstrap fix