Add Lawrence Esswood to list of contributors

Lawrence mostly defined the new 32-bit permissions format and has
co-authored multiple commits (and has been involved in the CHERIv9 design
in the past as well).

Author: arichardson

src/contributors.adoc

@@ -11,6 +11,7 @@ This RISC-V specification has been contributed to directly or indirectly by:
 * David Chisnall <david.chisnall@cl.cam.ac.uk>
 * Jessica Clarke <jessica.clarke@cl.cam.ac.uk>
 * Brooks Davis <brooks.davis@sri.com>
+* Lawrence Esswood <lesswood@google.com>
 * Nathaniel Wesley Filardo <nwf20@cam.ac.uk>
 * Franz A. Fuchs <franz.fuchs@cl.cam.ac.uk>
 * Timothy Hutt <timothy.hutt@codasip.com>