Add instructions for adding Retriever to the OS X path

Adding the path to a file in /etc/paths.d is the only approach
that works for both the command line and for RStudio. Symlinks
and .bash_profile do not work with RStudio, so this approach is
needed to support the ecoretriever R package.
Adding missing quotation mark to Mac path command
Merge remote-tracking branch 'upstream/master'

Conflicts:
	source/download.md