eos-s3: Add README for btn_counter example

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>

eos-s3/btn_counter/README.rst

@@ -0,0 +1,10 @@
+Button counter
+~~~~~~~~~~~~~~
+
+This example design features a simple 4-bit counter driving LEDs. To build the
+counter example, run the following command:
+
+.. code-block:: bash
+   :name: eos-s3-counter
+
+   make -C btn_counter