Add usage to script

Author: Jake-Shadle

physx-sys/make-sources.sh

@@ -1,6 +1,11 @@
 #!/bin/bash
 set -e
 
+# This script finds all of the cpp files in a <directory> and generates a rust
+# array of strings into physx-sys/sources/<name> that is used by the build script
+# to compile all of the source files when compiling physx-sys
+#
+# usage: make-sources.sh <physx c++ source directory path> <name>
 dir=$1
 file="$(dirname "$0")/sources/$2"