Update contributing.md with new integration process

Contributions will now be merged into an ARM Trusted Firmware
integration branch on GitHub instead of via an ARM internal
branch.
Add missing #include guard in xlat_tables.h

Change-Id: I7272a800accb7de71cbbf6b715a43061bbf79f8c
Merge pull request #34 from danh-arm/dh/new-integration-process

Update contributing.md with new integration process
Merge pull request #35 from sandrine-bailleux/sb/add-missing-include-guard

Add missing #include guard in xlat_tables.h