Clean up LICENSE file

We used our LICENSE file to describe both the license of the package,
and also the header template that should appear at the start of
all Java files we create.  This creates a confusing situation for
readers who just want to consume the package, because our file
header template starts off in the middle of a sentence.

Move our template header to a separate file, and reformat the text
of the license to be something more readable by a person reviewing
the project's terms of use.

Change-Id: If318e64c06683ea14e0240914c2d057c9199ce98
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>