doc: document windows shell support

explain that Git Bash and Cygwin require winpty to work correctly.
Added info on Git Bash running winpty automatically
when running node without .exe extension.

PR-URL: https://github.com/nodejs/node/pull/16104
Fixes: https://github.com/nodejs/node/issues/14100


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>