Added short clarification to colours of squares

this & previous pull requests close hotosm/learnosm#225 and closes hotosm/learnosm#224 as well as resolves hotosm/learnosm#201
Merge pull request #246 from Nick-Tallguy/gh-pages

Added new section, as draft, to beginner section of LearnOSM, dealing wi...
Merge pull request #248 from Nick-Tallguy/patch-1

Added short clarification to colours of squares