cosmetic: tweak README
cosmetic: tweak README

I noticed that the README is prominently shown on github, so it makes sense
to get rid of the worst typos and misinformation there.

Change-Id: I60defef6224a78bb84d7d0b57bc8da17ad7877bb

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts