Added additional documentation

Schema doc and utilities for maintaining/updating it.
Renamed some existing doc.
Created a new README in the documentation folder.
Created a new KNOWN_ISSUES doc to track SVG issues.
Fixed a bug in two of the OCA icons exports.
Re-exported to reflect the fixed OCA icons.
Merge pull request #139 from abouffard/master

Added additional documentation
Updated documentation README
Merge pull request #140 from abouffard/master

Updated documentation README
Simplified schema and tweaked some bits

Removed obsolete ContextAmplifiers and SpecialGraphic attributes
Improved documentation and svg READMEs
Changed air corridor symbols to lines
Updated affected csv files
Changed the location of the geometry exception tags
Changed the descriptions on the gct tags
Merge pull request #141 from abouffard/master

Simplified schema and tweaked some bits