Add Evan to authors.markdown
I have committed code to the main veracruz repo, so Dominic told me to add myself to the authors list.
Veracruz originated in the Systems (formerly Security) Research Group at Arm Research,
and is designed and written by the *Veracruz Development Team*:
+- Evan Johnson (evan.johnson@arm.com)
- Mathias Brossard (mathias.brossard@arm.com)
- Basma El Gaabouri (basma.elgaabouri@arm.com)
- Christopher Haster (christopher.haster@arm.com)
