developer guide enhancements.

CONTRIBUTING.md

@@ -34,16 +34,72 @@ the document that explains the way `sanic` manages dependencies inside the `setu
 | extras_require['dev']                     | Additional Development requirements to add contributing                    | pip3 install -e '.[dev]'     |
 | extras_require['docs']                    | Dependencies required to enable building and enhancing sanic documentation | pip3 install -e '.[docs]'    |
 
-## Running tests
-
-To run the tests for sanic it is recommended to use tox like so:
+## Running all tests 
+To run the tests for Sanic it is recommended to use tox like so:
 
 ```bash
 tox
 ```
-
 See it's that simple!
 
+`tox.ini` contains different environments. Running `tox` without any arguments will 
+run all unittests, perform lint and other checks. 
+
+## Run unittests : 
+`tox` environment -> `[testenv]`
+
+To execute only unittests, run `tox` with environment like so: 
+
+```bash
+tox -e py36 -v -- tests/test_config.py
+# or 
+tox -e py37 -v -- tests/test_config.py
+```
+
+## Run lint checks : 
+`tox` environment -> `[testenv:lint]`
+
+Permform `flake8`, `black` and `isort` checks.
+```bash
+tox -e lint
+```
+
+## Run other checks : 
+`tox` environment -> `[testenv:check]`
+
+Perform other checks.
+```bash
+tox -e check
+```
+
+# Code Style 
+To maintain the code consistency, Sanic uses following tools. 
+
+1. [isort](https://github.com/timothycrosley/isort)
+2. [black](https://github.com/python/black)
+2. [flake8](https://github.com/PyCQA/flake8)
+
+
+## isort
+`isort` sorts Python imports. It divides imports into three 
+categories sorted each in alphabetical order. 
+  1. built-in
+  2. third-party
+  3. project-specific
+
+## black
+`black` is a Python code formatter.
+
+## flake8
+`flake8` is a Python style guide that wraps following tools into one.
+1. PyFlakes
+2. pycodestyle
+3. Ned Batchelder's McCabe script 
+
+`isort`, `black` and `flake8` checks are performed during `tox` lint checks. 
+
+Refer [tox](https://tox.readthedocs.io/en/latest/index.html) documentation for more details.
+
 ## Pull requests!
 
 So the pull request approval rules are pretty simple: