Improve new dev experience in README (aws#674)

* Improve new dev experience in README

* Update README.rst

Author: andremoeller

CHANGELOG.rst

@@ -14,6 +14,7 @@ CHANGELOG
 * doc-fix: move overview content in main README into sphynx project
 * bug-fix: pass accelerator_type in ``deploy`` for REST API TFS ``Model``
 * doc-fix: move content from tf/README.rst into sphynx project
+* doc-fix: Improve new developer experience in README
 
 1.18.3.post1
 ============

README.rst

@@ -97,9 +97,16 @@ Running tests
 
 SageMaker Python SDK has unit tests and integration tests.
 
+You can install the libraries needed to run the tests by running :code:`pip install --upgrade .[test]` or, for Zsh users: :code:`pip install --upgrade .\[test\]`
+
 **Unit tests**
 
-tox is a prerequisite for running unit tests so you need to make sure you have it installed. To run the unit tests:
+
+We run unit tests with tox, which is a program that lets you run unit tests for multiple Python versions, and also make sure the
+code fits our style guidelines. We run tox with Python 2.7 and 3.6, so to run unit tests
+with the same configuration we do, you'll need to have interpreters for Python 2.7 and Python 3.6 installed.
+
+To run the unit tests with tox, run:
 
 ::
 
@@ -109,23 +116,29 @@ tox is a prerequisite for running unit tests so you need to make sure you have i
 
 To run the integration tests, the following prerequisites must be met
 
-1. Access to an AWS account to run the tests on
-2. AWS account credentials available to boto3 clients used in the tests
-3. The AWS account has an IAM role named :code:`SageMakerRole`
-4. The libraries listed in the ``extras_require`` object in ``setup.py`` for ``test`` are installed.
-   You can do this by running the following command: :code:`pip install --upgrade .[test]`
+1. AWS account credentials are available in the environment for the boto3 client to use.
+2. The AWS account has an IAM role named :code:`SageMakerRole` with the AmazonSageMakerFullAccess policy attached.
+
+We recommend selectively running just those integration tests you'd like to run. You can filter by individual test function names with:
+
+::
+
+    pytest -k 'test_i_care_about'
+
 
-You can run integ tests by issuing the following command:
+You can also run all of the integration tests by running the following command, which runs them in sequence, which may take a while:
 
 ::
 
     pytest tests/integ
 
-You can also filter by individual test function names (usable with any of the previous commands):
+
+You can also run them in parallel:
 
 ::
 
-    pytest -k 'test_i_care_about'
+    pytest -n auto tests/integ
+
 
 Building Sphinx docs
 ~~~~~~~~~~~~~~~~~~~~