Update hardware_donation_license.md

Noting detailed changes for this one because it's a formal doc...

1. Changed "extendible" to "exstensible" (See: http://english.stackexchange.com/questions/90426/extensible-vs-extendible/90444#90444).

2. Changed "XHDL" to "xHDL" – because I can. Also included "HDL" as it's likely to be commonly known.

3. Subtle rewording throughout.

4. Don't see point in the "Assumed Limitations of Application" section. Surely community-owned kit should be licensed like any private kit, or at least come under a blanket licensing stated by the community managers.

5. Added section "### A Little Background on xHDL"