[sw] Add a README.md to libbase
[sw] Add a README.md to libbase

A number of other commits add files to sw/device/lib/base, and
discussion around them indicates to a consensus that this directory
should house all of our "libc-like" stuff. This commit "officially"
creates the directory and gives it a README to codify this consensus.

Signed-off-by: Miguel Young de la Sota <71b8e7f4945fd97b98544cf897992af89646547a@google.com>

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