Documentation: Describe our ecosystem

Neither payloads nor distributors are an integral part of the coreboot
source tree, but they're very important parts of the coreboot
ecosystems, so add some descriptions.

Change-Id: Id64744c252b6b78c4811fbded48c441ef486ad94
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/31180
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

Documentation/index.md

@@ -13,6 +13,8 @@ Contents:
 * [Code of Conduct](community/code_of_conduct.md)
 * [Community forums](community/forums.md)
 * [coreboot at conferences](community/conferences.md)
+* [Payloads](payloads.md)
+* [Distributions](distributions.md)
 * [Timestamps](timestamp.md)
 * [Intel IFD Binary Extraction](Binary_Extraction.md)
 * [Dealing with Untrusted Input in SMM](technotes/2017-02-dealing-with-untrusted-input-in-smm.md)