'make clean' removes OASIS cache

This is important if you want to switch compiler versions and
then 'make clean && make'

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Improve the README.md

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
git ignore *.native (OASIS binary symlinks)

Signed-off-by: David Scott <dave.scott@eu.citrix.com>