readme: remove some trailing whitespace

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
Spring: use constant for Bundle-Blueprint header

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
Project: use the correct properties for get{Test{Src,Output},Target0}

And add FIXMEs

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
Project: get the default values from defaults.bnd

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
Project: use constants for the src/bin/testsrc/testbin/target-dir properties

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
Project: expose accessors for src/bin/testsrc/testbin/target-dir

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>