Add a README

This gives a short introduction and clarifies the licensing.

README.md

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+Atmel Lightweight Mesh for Arduino
+----------------------------------
+This is an Arduino port of the Atmel LWM network stack. Most of the code
+is taken as-is, with some small tweaks to make it work with the Arduino
+stack.
+
+Original code from atmel can be downloaded here: http://www.atmel.com/tools/lightweight_mesh.aspx
+
+The upstream branch contains only sources directly taken from the
+upstream zipfile, using the import script in this repository. The only
+modifications made are done using that import script.
+
+The master branch contains the same sources, with additions to make them
+compile in the Arduino environment. The pinoccio branch contains some
+modifications specific to the [pinocc.io][] software stack.
+
+[pinocc.io]: http://www.pinocc.io
+
+License
+-------
+The code from Atmel is licensed under a custom open source license,
+as found in license.txt (but not without restrictions, so be sure to
+really look at the licensing terms). This is the same license as
+presented during the download process.
+
+Note that comments in the source files indicate another, shorter and slightly
+different license ("asf license"), but also refer to license.txt so the exact
+terms are not entirely clear.  When asked, Atmel replied on 2014-03-04
+that the license.txt is binding. In version 1.2.1 (after my question),
+they added a reference to license.txt in all source file headers, but
+did not remove the "superseded" asf license, which makes things still
+slightly confusing.
+
+> > (Me) In particular, there is a file called "license.txt", which contains a
+> > "LIMITED LICENSE AGREEMENT" that grants fairly liberal rights to the
+> > Software. However, looking inside the actual source files, I see that
+> > most of them contain a header with a different and slightly more
+> > libraral license (referred to as "asf\_license", which I take to mean
+> > Atmel Software Framework?).
+> >
+> > (Me) Could you clarify what the intent here is? In particular, I'd assume
+> > that the license in the header of a file applies to the content of that
+> > file, and needs to be retained when reusing or redistributing (parts of)
+> > that file. However, does the longer license in license.txt also apply
+> > and do I need to keep that one distributed as well?
+
+> (Atmel) The license.txt file supersedes the asf\_license (Atmel Software Framework
+> License).
+
+> > (Me) However, does the longer license in license.txt also apply and do I need to
+> > keep that one distributed as well?
+
+> (Atmel) Yes, license.txt should be distributed.
+
+The import script is licensed under a more liberal license, see the file itself
+for details.