Documentation refactor and updates.

 * Includes updating ContributorGuide.md with information section
   'Contributing code to Arm NN'.
 * Renaming of ContributorGuide.md to CONTRIBUTING.md. Also renamed
   associated licence file and links in other pages that call the md file.

Signed-off-by: Cathal Corbett <cathal.corbett@arm.com>
Change-Id: Ic3336d78b64ae41af07ea9fd03631bb161613c4b
diff --git a/CONTRIBUTING.md.license b/CONTRIBUTING.md.license
new file mode 100644
index 0000000..739dc2d
--- /dev/null
+++ b/CONTRIBUTING.md.license
@@ -0,0 +1,4 @@
+#
+# Copyright © 2018-2022 Arm Ltd and Contributors. All rights reserved.
+# SPDX-License-Identifier: MIT
+#