issue #55 Update README with make docs command

Use Makefile for docs and tests

Makefile

+.PHONY: docs tests
+
+# generate documentation
+docs:
+	cd docs && sphinx-apidoc -o . ../duniterpy && make html && cd ..
+
+# run tests
+tests:
+	python -m unittest
+

README.rst

@@ -46,7 +46,7 @@ Development
 * Add PYTHONPATH env var to your shell containing the path to this repository
 * Run unit tests with::
 
-    python -m unittest
+    make tests
 
 * Take a look at examples folder
 * Run examples from parent folder::
@@ -60,6 +60,10 @@ Documentation
 
     pip install -r requirements_dev.txt
 
+* Generate documentation::
+
+    make docs
+
 * HTML Documentation is generated in docs/_build/html folder.
 * `Examples <https://git.duniter.org/clients/python/duniterpy/tree/master/examples>`_