Clarified instructions on "Use a different source repository"

[dougtoppin]

In the README "Use a different source repository" section I had to have my presentations be in a directory called "presentations/".

This is because present.py check_custom() copies to BASE + src but PRES is expecting them to be in BASE + 'src/presentations'

Otherwise when I ran the container I would get this error:

find: `/opt/revealjs/src/presentations/*': No such file or directory

Available Presentations
=======================

find: `/opt/revealjs/src/presentations/*': No such file or directory

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