Add a contributors file to provide a high level overview
for people who wish to contribute to the project outlining
basic details and setting some project expectations.

This isn't intended to replace any of the existing documentation
but rather provide a succinct top level document that's easy
to find to enable users to understand the project and get
started as quickly as possible.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
[trini: Correct merge window length, release day and typo in the main
        index]
Signed-off-by: Tom Rini <trini@konsulko.com>