Add some documentation

Author: jerry-git

README.md

@@ -1 +1,40 @@
 # Pytest-split
+
+[![Build Status](https://travis-ci.org/jerry-git/pytest-split.svg?branch=master)](https://travis-ci.org/jerry-git/pytest-split)
+[![PyPI version](https://badge.fury.io/py/pytest-split.svg)](https://pypi.python.org/pypi/pytest-split/)
+[![PyPI pyversions](https://img.shields.io/pypi/pyversions/pytest-split.svg)](https://pypi.python.org/pypi/pytest-split/)
+
+Pytest plugin which splits the test suite to equally sized "sub suites" based on test execution time.
+
+## Motivation
+* Splitting the test suite is a prerequisite for parallelization (who does not want faster CI builds?). It's valuable to have sub suites which execution time is around the same.  
+* [`pytest-test-groups`](https://pypi.org/project/pytest-test-groups/) is great but it does not take into account the execution time of sub suites which can lead to notably unbalanced execution times between the sub suites.
+* [`pytest-xdist`](https://pypi.org/project/pytest-xdist/) is great but it's not suitable for all use cases.
+For example, some test suites may be fragile considering the order in which the tests are executed.
+This is of course a fundamental problem in the suite itself but sometimes it's not worth the effort to refactor, especially if the suite is huge (and smells a bit like legacy).
+Additionally, `pytest-split` may be a better fit in some use cases considering distributed execution.
+
+## Installation
+```
+pip install pytest-split
+```
+
+## Usage
+First we have to store test durations from a complete test suite run.
+This produces .test_durations file which should be stored in the repo in order to have it available during future test runs. 
+The file path is configurable via `--durations-path` CLI option.
+```
+pytest --store-durations
+```
+
+Then we can have as many splits as we want:
+```
+pytest --splits 3 --group 1
+pytest --splits 3 --group 2
+pytest --splits 3 --group 3
+```
+
+Time goes by, new tests are added and old ones are removed/renamed during development. No worries!
+`pytest-split` assumes average test execution time (calculated based on the stored information) for every test which does not have duration information stored.
+Thus, there's no need to store durations after changing the test suite.
+However, when there are major changes in the suite compared to what's stored in .test_durations, it's recommended to update the duration information with `--store-durations` to ensure that the splitting is in balance.