README.md: improve contribution text, add Augie image
README.md: improve contribution text, add Augie image

Change-Id: I8e9c746b8f112b7ea6a8901784a5ab9d8fea0121
Reviewed-on: https://upspin-review.googlesource.com/7924
Reviewed-by: David Presotto <516b9783fca517eecbd1d064da2d165310b19759@google.com>
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@google.com>

upspin/upspin,upspin/upspin