Add testing/build steps to the README

[abatilo]

Every time we receive a PR, we run the following steps:

sanic-swagger/.circleci/config.yml

Lines 3 to 31 in ca16ca4

	lint_git_commits:
	docker:
	- image: python:3.6
	working_directory: ~/sanic-swagger
	steps:
	- checkout
	- run: pip install gitlint
	- run: gitlint --commits fbb925a...HEAD
	lint:
	docker:
	- image: python:3.6
	working_directory: ~/sanic-swagger
	steps:
	- checkout
	- run: pip install pipenv
	- run: pipenv install --dev
	- run: pipenv run flake8 --show-source --import-order-style pep8
	test:
	docker:
	- image: python:3.6
	working_directory: ~/sanic-swagger
	steps:
	- checkout
	- run: pip install pipenv
	- run: pipenv install --dev
	- run: pipenv run pytest --cov-report html --cov-report xml:codecov.xml --junit-xml=coverage.xml --cov-branch --cov-fail-under=90 -v --cov=sanic_swagger tests/
	- run: mkdir -p test-results/pytest && cp coverage.xml test-results/pytest/results.xml

It would be extremely useful to have sections in the README for how to run these locally, so that people who want to contribute, can make sure that all of these steps will pass locally, before pushing the PR.