Devel - please pull/merge these changes to the xrdp-genkeymap utility

I noticed on the xrdp-devel list that someone was having keymap trouble because xrdp-keygen doesn't generate a [shiftaltgr] section.

I am hoping this change to genkeymap.c now fixes that.

I also made some changes to the readme file both to reflect the change to the utility, and some spelling, grammar, and formatting issues.