Add readme for the hello-world example

Closes #1070.

examples/hello-world/README.md

@@ -0,0 +1,21 @@
+Hello World!
+============
+
+This is a basic example how to use RIOT in your embedded application.
+It prints out the famous text `Hello World!`.
+
+This example should foremost give you an overview how to use the Makefile system:
+
+* First you must give your application a name, which is commonly the same as the name of the directory it resides in.
+  Then you can define a default BOARD for which the application was written.
+  By using e.g. `make BOARD=msba2` you can override the default board.
+  With `make buildtest` the application gets compiled for all supported boards.
+
+* The variable `RIOTBASE` contains an absolute or relative path to the directory where you have checked out RIOT.
+  If your code resides in a subdirectory of RIOT, then you can use `$(CURDIR)` as it's done in here.
+
+* The variable `QUIET`, which is either `1` or `0`, defines whether to print verbose compile information, or hide them, respectively.
+
+* The last line of your Makefile must be `include $(RIOTBASE)/Makefile.include`.
+
+The code itself may look like your usual *C* beginners hello-world example.