Update link to changelog 2.1
minor fix
add known issues

add text of wiki page known issues
Merge pull request #197 from jonnyforestGIS/master

add known issues text
minor link fix
housekeeping in preamble files
housekeeping in introduction docs (still need some work)
more housekeeping: browser and projections files
cleanup and extent supported vector data section
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
cleanup (big one)
update whats new for 2.2 release
Merge branch 'master' of github.com:qgis/QGIS-Documentation