Adding a link to Gernot's blog post that explains what GPL on seL4
means for other code. This is mainly intended for people who aren't
that familiar with what all of these licenses mean.
Closes#524
Signed-off-by: Gerwin Klein <gerwin.klein@proofcraft.systems>
github detects LICENSE.md files and links to them, so put the license
information there instead of the README.
Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>