Add minimum required git version in README

[pawamoy]

As seen in issue #18, minimum git version required is 2.6.0.

[mhagger]

@pawamoy: Thanks for this patch. Did you actually verify that git-sizer runs with v2.6.0? (I.e., are there any newer features that it relies on?) I don't think so, but it would be good to check.

Long term we could ask git what version it is and, if it is less than 2.6.0, could omit the --buffer option (which is only there for efficiency, not correctness).

[pawamoy]

I did not try with git == 2.6.0 indeed, but latest (2.16.2). Just let me try and I'll come back to tell you!

[pawamoy]

So I tried with git 2.5.0, 2.5.5 and 2.6.0. Only git 2.6.0 made git-sizer happy.

If git-sizer can support older versions than 2.6.0, then it is even better, and you can close this PR 😄

[mhagger]

So I tried with git 2.5.0, 2.5.5 and 2.6.0. Only git 2.6.0 made git-sizer happy.

Awesome, thanks @pawamoy ✨

[larsxschneider]

Maybe you could reuse Git LFS code to check the Git version? https://github.com/git-lfs/git-lfs/blob/98288ae18899244501850a92065b207ce18821f7/git/version.go#L32