Update .../init.d/README.txt to clarify filenaming rules

[IngwiePhoenix]

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Updates the description in .../init.d/README.txt to slightly clarify filenaming rules to not include .sh/.bash and to exit properly.

I ran into this while trying to make VirtualHere's USB server launch on startup and had .sh at the end of my filename, thus it never launched. So, I added the missing clarifications just in case so others hopefuly don't bump into this little caveat! :)