Add copyright note to README

We already specify the license in the source code, but just to be clear
we add a note to README that spells it out. Clarify in README that the
license is ISC and that the details can be found in COPYING.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Add installation notes

Create a file with notes that explains how to build and install the
software.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>