README: Clarify red-black tree statements.

README: Clarify red-black tree statements.

As noted by Sebastian Stark, the old text was a bit misleading. So
make it clear that no special linux files are necessary to compile
the library.

Also, the sentence on table locking was not quite true: There's the
dirty bit in each osl table which is some kind of locking. Correct that
statement as well.