Add header to local sdk files, update contribution notes

[monde]

• Add header // DO NOT EDIT LOCAL SDK - USE v3 okta-sdk-golang FOR API CALLS THAT DO NOT EXIST IN LOCAL SDK to all golang files in the local SDK directory
• add sdk/README.md file explaining the nature of the local sdk
• Concisely explain our minimum requirements for bug fixes and new resources and data sources directly in the main project README.md

for f in `find sdk -type f`; do
  vim -c "normal! gg" -c "normal! O// DO NOT EDIT LOCAL SDK - USE v3 okta-sdk-golang FOR API CALLS THAT DO NOT EXIST IN LOCAL SDK" -c x ${f}
done