README: Use absolute link for 'Recent changes', enabling use on PyPI.

While the links at the top of the README do not currently work on PyPI,
most point within the README document and so can be found on the PyPI
page. However, until this commit, since the 'Recent changes'
(CHANGELOG.md) point to another document on github then they are much
less visible, and are likely of interest if people are checking the
package history on PyPI.

Author: neiljp

README.md

@@ -2,7 +2,7 @@
 
 An interactive terminal interface for [Zulip](https://zulipchat.com).
 
-[Recent changes](CHANGELOG.md) | [Installation & Running](#Installation--running) | [Hot Keys](#hot-keys) | [Troubleshooting](#troubleshooting-common-issues) | [Development](#contributor-guidelines)
+[Recent changes](https://github.com/zulip/zulip-terminal/blob/master/CHANGELOG.md) | [Installation & Running](#Installation--running) | [Hot Keys](#hot-keys) | [Troubleshooting](#troubleshooting-common-issues) | [Development](#contributor-guidelines)
 
 [![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)