* Clarify differences between BTU mod V3 and V4
- Added description
- V4 has now been tested so removed warning
- Added better image of v3
- Added image of v4
* Update printing and assembly recommendations
- The repo's wiki has much better 3D printing recommendations
- Previously was missing any mention of the M3 threaded inserts which
should be used. Now added and the wiki is also linked for those.
George Bryant