Improve installation instructions

1. Many users (including me) are running into the executable bit trap (which isn't obvious on Windows).
1. There's no usage example for the new --reuse option yet.

Regarding the latter, it actually took me some time to get ssh-pageant working with both Cygwin and MinGW (msysgit) at the same time, sharing a single socket.

The instructions added here are just a baseline, which might help new users to set it up more quickly.

In my particular case, msysgit did still not actually work, even after figuring out the socket file path hiccup (resorting to $TEMP fixed that) -- TortoiseGit/msysgit actually seems to ship with a OpenSSH/OpenSSL client version that is not compatible with ssh-pageant...

After lots of trial and error, I managed to get it working by making the MinGW bash force-use more current binaries from Cygwin, by putting the following at the top of `.bashrc`:
```sh
export PATH=/c/cygwin/bin:${PATH}
```

However, I don't know whether that is worth to document, as I don't know how common the TortoiseGit/MinGW + Cygwin combination is in the wild.  I suspect it is quite common, but YMMV.

ssh-pageant seems to work correctly now - I only see one process running, regardless of how many shells I'm invoking (and regardless of provider).

What do you think?  Is the example correct?

In addition, I considered to remove the other existing example, since the new --reuse option definitely appears to be preferable and is simpler to set up on top.  But didn't touch that, so as to keep changes minimal.

Lastly, I think ssh-pageant is really useful, and would offer you to rewrite/redo `README` and `INSTALL` into a new and combined `README.md` file that encompasses everything new users ought to know.  I'm aware that the separate files are common in *nix, but the primary "homepage" of your nice tool seems to be GitHub, and thus, the first thing that every potential new user reads is the readme → the simpler, shorter, better the readme, the more happy users. ;-)  If you agree, I'll create a (separate) PR for that.

Thank you for developing this excellent piece of code!