docs/contributing: move installation of dev_requirements to default instructions

When intending to contribute to Kallithea, you are expected to run all
tests, so installation of the dev_requirements are actually mandatory.
Hence, update the instructions.

docs/contributing.rst

@@ -36,6 +36,7 b' To get started with Kallithea developmen'
         source ../kallithea-venv/bin/activate
         pip install --upgrade pip setuptools
         pip install -e .
+        pip install -r dev_requirements.txt
         gearbox make-config my.ini
         gearbox setup-db -c my.ini --user=user --email=[e-mail] --password=password --repos=/tmp
         gearbox serve -c my.ini --reload &
@@ -81,11 +82,9 b' mailing list (via ``hg email``) or by cr'
 Running tests
 -------------
 
-After finishing your changes make sure all tests pass cleanly. Install the test
-dependencies, then run the testsuite by invoking ``py.test`` from the
-project root::
+After finishing your changes make sure all tests pass cleanly. Run the testsuite
+by invoking ``py.test`` from the project root::
 
-    pip install -r dev_requirements.txt
     py.test
 
 Note that testing on Python 2.6 also requires ``unittest2``.