Add tox infos

Signed-off-by: Stephen L. <lrq3000@gmail.com>

CONTRIBUTE

@@ -36,7 +36,7 @@ TESTING
 To test functionality on your machine (such as before submitting a Pull
 Request), there are a number of unit tests.
 
-To run the tests,
+To run the tests for only your Python distribution:
 
 - install `tox`
 - `cd` to the root of the `tqdm` directory (in the same folder as this file)
@@ -56,5 +56,16 @@ nosetests --with-coverage --cover-package=tqdm -v tqdm/
 python -m flake8 tqdm/_tqdm.py
 ```
 
+To run the tests for all Python distributions (better, but more time consuming):
+
+- install `tox`
+- install all versions of the Python interpreter that are specified in
+[tox.ini](tox.ini). You can use MiniConda to install a minimal setup.
+You must also make sure that each distribution has an alias to call
+the Python interpreter: python27 for Python 2.7's interpreter,
+python32 for Python 3.2's, etc.
+- `cd` to the root of the `tqdm` directory (in the same folder as this file)
+- run the following command: `tox`
+
 Then wait for the tests to finish, and check your console to see if there's
 any issue.

Makefile

@@ -16,6 +16,8 @@ testsetup:
 testcoverage:
 	nosetests tqdm --with-coverage --cover-package=tqdm -v
 
+allenvtests: tox
+
 installdev:
 	python setup.py develop --uninstall
 	python setup.py develop