Merge pull request #299 from isuruf/authors

Add Richard and Erik to AUTHORS. Welcome to SymEngine!!

Author: isuruf

AUTHORS

@@ -23,3 +23,5 @@ xoviat <xoviat@users.noreply.github.com>
 Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>
 Moritz E. Beber <morbeb@biosustain.dtu.dk>
 Alan Hu <alanlh2@illinois.edu>
+Richard Otis <richard.otis@outlook.com>
+Erik Jansson Agnvall <erikjansson90@gmail.com>