Mention ARM compatibility explicitly in the README

[m040601]

Like many others in the last years,
#55
#41

I also can attest that gotty works perfectly in
various arm architectures, armv5 armv6 v7 etc.

They match various popular devices like the various Raspberry models, Pi2 Pi3 PiZero.

The release page, listing the binaries,
https://github.com/yudai/gotty/releases
simply states "linux_arm.tar.gz" making no explicit version of what exact arm version.

I understand that, as found in,

yudai commented on Sep 5, 2015
.... it's hard to maintain binaries for all ARM versions by default. ...and I don't want to make the file list in the releases page longer by supporting those different versions....

and that:

....using ARMv5 as the default ....

Could you add this information to the README page ?
Thanks in advance