docs: better clarify the build environment
CONTRIBUTING.md and ./build now explain the requirements better.
@@ -263,7 +263,8 @@ clean) shift; clean;;
##
####
help) shift; help "$@";;
-## install Copy ockam binary to /usr/local/bin/ockam.
+## install Copy ockam binary to /usr/local/bin/ockam, for OSX
+## or Linux environments.
##
## Examples:
## ./build install
