Add some top level reviewers (#964)

Author: Josh Crowther

.github/CODEOWNERS

@@ -2,7 +2,7 @@
 # Each line is a file pattern followed by one or more owners.
 
 # These owners will be the default owners for everything in the repo.
-*       @jshcrowthe @hiranya911
+*       @jshcrowthe @hiranya911 @mikelehen @bojeil-google @depoll
 
 # Database Code
 packages/database  @mikelehen @schmidt-sebastian