README: Add GitHub Action badge for linting and tests.

Note that this does not remove the Travis badge.

Author: neiljp

README.md

@@ -5,7 +5,8 @@
 [![Zulip chat](https://img.shields.io/badge/zulip-join_chat-brightgreen.svg)](https://chat.zulip.org/#narrow/stream/206-zulip-terminal)
 [![PyPI](https://img.shields.io/pypi/v/zulip-term.svg)](https://pypi.python.org/pypi/zulip-term)
 [![Python Versions](https://img.shields.io/pypi/pyversions/zulip-term.svg)](https://pypi.python.org/pypi/zulip-term)
-[![Build Status](https://travis-ci.com/zulip/zulip-terminal.svg?branch=master)](https://travis-ci.com/github/zulip/zulip-terminal)
+[![Travis Build Status](https://travis-ci.com/zulip/zulip-terminal.svg?branch=master)](https://travis-ci.com/github/zulip/zulip-terminal)
+[![GitHub Actions - Linting & tests](https://github.com/zulip/zulip-terminal/workflows/Linting%20%26%20tests/badge.svg?branch=master)](https://github.com/zulip/zulip-terminal/actions?query=workflow%3A%22Linting+%26+tests%22+branch%3Amaster)
 [![Coverage status](https://img.shields.io/codecov/c/github/zulip/zulip-terminal/master.svg)](https://codecov.io/gh/zulip/zulip-terminal)
 [![Checked with mypy](http://www.mypy-lang.org/static/mypy_badge.svg)](http://mypy-lang.org/)