Update_freebsd_install_instructions.mdwn

### Please describe the problem.

The install documentation is out of date at least for the FreeBSD example.

### Please provide any additional information below.

Latest FreeBSD 10.x uses pkgng instead of pkg_add and the correct command is now 'pkg install hs-git-annex'.

If you find the time please update it at https://git-annex.branchable.com/install/ .

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