Merge pull request https://github.com/RIOT-OS/applications/pull/58 from miri64/readme

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)
+
+# RIOT Applications
+
+This repository provides more applications for the [RIOT operating system][riot-repo].
+Some of them are just useful tools for development work, others showcase
+more extensive implementations of features of RIOT compared to the rather simple
+[examples in the RIOT main codebase][riot-repo/examples].
+
+## Usage
+
+To build and use them follow [the instructions in the RIOT repository][getting-started]
+and the READMEs within the respective application directory.
+
+The RIOT main code is included as a submodule. This always points 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 git@github.com:RIOT-OS/RIOT.git
+cd applications
+RIOTBASE="../RIOT" BOARD=samr21-xpro make -C sniffer flash
+```
+
+Alternatively, you can step into the submodule and check out the current master:
+
+```sh
+cd RIOT
+git checkout master
+git pull
+```
+
+Note that there is no guarantee that it will build or work, since we only test
+this repository against the latest release.
+
+[riot-repo]: https://github.com/RIOT-OS/RIOT
+[riot-repo/examples]: https://github.com/RIOT-OS/RIOT/tree/master/examples
+[getting-started]: https://github.com/RIOT-OS/RIOT/blob/master/README.md#getting-started