Add alt text to badges so that they're accessible

Author: jturner314

README.rst

@@ -12,15 +12,19 @@ __ https://docs.rs/ndarray/
 |build_status|_ |crates|_ |matrix-chat|_ |irc|_
 
 .. |build_status| image:: https://github.com/rust-ndarray/ndarray/workflows/Continuous%20integration/badge.svg?branch=master
+   :alt: CI build status
 .. _build_status: https://github.com/rust-ndarray/ndarray/actions
 
 .. |crates| image:: http://meritbadge.herokuapp.com/ndarray
+   :alt: ndarray at crates.io
 .. _crates: https://crates.io/crates/ndarray
 
 .. |matrix-chat| image:: https://img.shields.io/badge/Matrix-%23rust--sci%3Amatrix.org-lightgrey
+   :alt: Matrix chat at #rust-sci:matrix.org
 .. _matrix-chat: https://matrix.to/#/#rust-sci:matrix.org
 
 .. |irc| image:: https://img.shields.io/badge/IRC-%23rust--sci%20on%20OFTC-lightgrey
+   :alt: IRC at #rust-sci on OFTC
 .. _irc: https://webchat.oftc.net/?channels=rust-sci
 
 Highlights