Improve INSTALL markdown formatting

[wilcoxjay]

When reading the INSTALL file as rendered on github, a few things are not as nice as they could be.

• The installation instructions rendered so that all the shell commands were run together as normal text. I added additional spaces at the beginning of each line to convince github that the shell commands are code.
• The file began with an emacs file variable line, which confused the renderer. Interestingly, simply deleting the line causes the file to be rendered as plain text. So I additionally renamed the file to end with a .md suffix. This has the unfortunate side effect of breaking existing references to the file (such as on the wiki where I originally found a reference to the INSTALL file, but probably there are others as well).

I realize that the original intention of this file was probably not to be read on github. But it is referred to by the wiki, and since the README is formatted so nicely for github, it seems like INSTALL should be, too.

[msftclas]

This seems like a small (but important) contribution, so no Contribution License Agreement is required at this point. We will now review your pull request.
Thanks,
Microsoft Pull Request Bot

[cpitclaudel]

Brilliant, thanks :)