Add to cl_khr_command_buffer contributors (#754)

In an oversight I missed out some individuals who contributed to the
internal extension MR(#180) and issue(#251).

I've gone through the GitLab participants list of both to these and
ensured everyone is now present under contributors.

Author: Ewan Crawford

ext/cl_khr_command_buffer.asciidoc

@@ -30,6 +30,7 @@ This extension requires OpenCL 1.2 or later.
 ==== Contributors
 
 Ewan Crawford, Codeplay Software Ltd. +
+Gordon Brown, Codeplay Software Ltd. +
 Kenneth Benzie, Codeplay Software Ltd. +
 Alastair Murray, Codeplay Software Ltd. +
 Jack Frankland, Codeplay Software Ltd. +
@@ -39,6 +40,9 @@ Kevin Petit, Arm Ltd. +
 Aharon Abramson, Intel. +
 Ben Ashbaugh, Intel. +
 Boaz Ouriel, Intel. +
+Pekka Jääskeläinen, Tampere University +
+Nikhil Joshi, NVIDIA +
+James Price, Google +
 
 === Overview