Add RIOT OS prototype readme and WoT TD

[Citrullin]

I added some information about the RIOT OS WoT TD implementation. It includes a how to.
Not sure, if it should be in prototypes or products though.

[mmccool]

Let's discuss in the call today. Probably we should mark this as inactive for now, since among other possible issues, I notice only a relative href is given in the TD but there is also no "base" given. Not sure it would actually fail validation given our current tests, but it's not actually usable since among other issues, it's not possible to determine what protocol you are using (I'm guessing CoAP since you include that vocabulary, but there is still not a full URL available).

One option would be to merge it but to rename the TD as a ".json" file and/or mark it as inactive in "active.csv".

[mmccool]

Discussed in plugfest call, agreed no base and rel url is technically allowed, but a corner case (what is the protocol?). Could be a Thing Model (TM), eg using a URL template for the base, but if a TM is checked in as a ".jsonld" file it will complicate validation. Suggest TMs get checked in as ".tm-jsonld"

[relu91]

".jsonld" file it will complicate validation. Suggest TMs get checked in as ".tm-jsonld"

Using tm-jsonld it is a little bit problematic because it turns off the automatic format inference of most IDEs (although you can manually set it). I suggest to use td.jsonld for TDs and tm.jsonld for TMs. BTW td-code already use td.json and td.jsonld for validation.