added nautical miles and degrees for measurements units
update info about item ID from Andreas Neumann - thanks
add note on packages, pip and virtual envs. Add build tools header.
add "Paste into new layer"
Merge pull request #201 from mewoch/man2-2

Man2 2
corrected description of update preview
Merging new training manual into master
linking to training manual
Merge branch 'master' of https://github.com/qgis/QGIS-Documentation