Update README

README.rst

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-python-cloud-tasks-deferred
-===========================
+cloud-tasks-deferred
+====================
 
-An unfinished deferred library for Google Cloud Tasks.
+.. image:: https://img.shields.io/pypi/v/cloud-tasks-deferred.svg
+   :alt: PyPI
+   :target: https://pypi.org/project/cloud-tasks-deferred
+
+.. image:: https://img.shields.io/travis/grktsh/python-cloud-tasks-deferred/master.svg
+   :alt: Travis
+   :target: https://travis-ci.org/grktsh/python-cloud-tasks-deferred
+
+.. image:: https://img.shields.io/codecov/c/github/grktsh/python-cloud-tasks-deferred/master.svg
+   :alt: Codecov
+   :target: https://codecov.io/gh/grktsh/python-cloud-tasks-deferred
+
+A deferred library for Google Cloud Tasks.
+
+Prerequisites
+-------------
+
+- Environment variable ``QUEUE_LOCATION`` in which Cloud Tasks service runs
+
+Unavaiable options
+------------------
+
+- ``_transactional``: Transactional tasks are unavailable in Cloud Tasks
+- ``_retry_options``: Retry options for tasks are unavailable in Cloud Tasks
+
+Unimplemented features
+----------------------
+
+- Support for tasks larger than 10 KB