Update OS X word selection shortcuts

Clarified Command + D; Added Ctrl + Command + G
Merge pull request #104 from iainhouston/patch-1

Update OS X word selection shortcuts
add metadata topics
minor fixes
edit metadata topic
edit
split lines
Merge pull request #109 from guillermooo/build-systems

split lines
edit metadata topics

more x-refs, code highlighting for xml, properly indented code blocks,
various other adjustments (including rephrasing some sections)
Also add a "seealso" block to the symbols doc
add backers list
Merge pull request #107 from guillermooo/backers-list

add list of backers
edit README
Merge pull request #108 from guillermooo/add-bs-donation-link

edit README
add 'backers' section
Merge pull request #112 from guillermooo/backers-list-2

add 'backers' section
Address feedback from 7bf1ec38 (#102)
"document" -> "page"
Very minor adjustments

From ccb7f2b feedback.
Merge pull request #102 from guillermooo/new-topic-metadata

comments, symbols, metadata