admin(README.md): add @b-mehta to the maintainers list (#6986)

Co-authored-by: Scott Morrison <scott.morrison@gmail.com>
Co-authored-by: Bhavik Mehta <bhavikmehta8@gmail.com>

README.md

@@ -64,5 +64,6 @@ welcomed.
 * Robert Y. Lewis (@robertylewis): all
 * Heather Macbeth (@hrmacbeth): geometry, analysis
 * Patrick Massot (@patrickmassot): documentation, topology
+* Bhavik Mehta (@b-mehta): category theory, combinatorics
 * Scott Morrison (@semorrison): category theory
 * Eric Wieser (@eric-wieser): algebra, infrastructure