Edit .bashrc commands and add closing comments

The installation section mentioned the following command to make the
~/povray/man available in the search path for manual pages.

    export MANPATH=$PATH:~/povray/man

While this command made the POV-Ray manual page available in the search
path, it made all other manual pages unavailable. For example, `man ls`,
`man cp`, etc.  stopped working. The following is the correct command
that makes the POV-Ray manual page available while keeping all existing
manual pages available in the search page for manual pages.

    export MANPATH=:~/povray/man

While this command is correct, it is redundant to add this in ~/.bashrc
because there is another command in ~/.bashrc that adds ~/povray/bin to
the PATH environment variable. This automatically makes ~/povray/man
available in the search path for manual pages. Therefore, remove the
MANPATH export command from README.md.

Also, add closing comments for May 2013 challenge.