Add descriptions to tox environments

With that we can expose the contributer to `tox -av` in the contributers
guidelines.

docs/src/contributing.rst

@@ -26,7 +26,12 @@ Then this package itself
   pip install -e .
 
 This install the package in development mode, making it importable globally and allowing
-you to edit the code and directly use the updated version.
+you to edit the code and directly use the updated version. To see a list of all
+supported tox environments please use
+
+.. code-block:: bash
+
+  tox -av
 
 Running the tests
 -----------------

tox.ini

@@ -12,7 +12,7 @@ lint_folders =
 
 
 [testenv:build]
-# builds the package and checks integrity
+description = Builds the package and checks integrity
 
 usedevelop = true
 deps =
@@ -29,6 +29,7 @@ commands =
     check-manifest {toxinidir}
 
 [testenv:tests]
+description = Runs the tests
 usedevelop = true
 changedir = tests
 deps =
@@ -51,6 +52,7 @@ commands_post =
     coverage xml
 
 [testenv:lint]
+description = Checks the code and doc for programmatic and stylistic errors
 skip_install = true
 deps =
     black
@@ -87,6 +89,7 @@ commands =
     isort {[tox]lint_folders}
 
 [testenv:docs]
+description = Builds the documentation
 usedevelop = true
 deps =
     -r docs/requirements.txt