Documentation: Add markdown license information

Add CC-by 4.0 license written in markdown and add global copyright string.

Change-Id: I31dc540d63b289d38d6d7d9a7c35b6bb50f9c92d
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/26264
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>

Documentation/getting_started/index.md

@@ -4,3 +4,4 @@
 * [Submodules](submodules.md)
 * [Kconfig](kconfig.md)
 * [Gerrit Guidelines](gerrit_guidelines.md)
+* [Documentation License](license.md)