Provide README

README.md

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+[![Build Status](https://travis-ci.org/RIOT-OS/applications.svg?branch=master)](https://travis-ci.org/RIOT-OS/applications)
+
+This repository provides some applications for the [RIOT operating system][riot-repo]. Some of them are just useful tools for
+development work, some showcase software provided with RIOT beyond the simple
+examples within the RIOT codebase.
+
+To build and use them follow [the instructions in the RIOT repository][getting-started]
+and the respective application's README.
+
+The submodule to the RIOT repository in this repository will always point to the
+latest release. To change the RIOT version to build against (e.g. current
+master), clone the RIOT repository in a separate repository and point the
+`RIOTBASE` environment variable there:
+
+```sh
+# assuming you are in the working directory of your local clone of this repo
+cd ..
+git clone [email protected]:RIOT-OS/RIOT.git
+cd applications
+RIOTBASE="../RIOT" BOARD=samr21-xpro make -C sniffer flash
+```
+
+[riot-repo]: https://github.com/RIOT-OS/RIOT
+[getting-started]: https://github.com/RIOT-OS/RIOT/blob/master/README.md#getting-started