Change in coreboot[master]: Documentation: Update index.md and add 4.18 release notes

[Felix Singer (Code Review)]

Attention is currently required from: Martin L Roth, Julius Werner.
Felix Singer has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/64843 )

Change subject: Documentation: Update index.md and add 4.18 release notes
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Patch Set 1:

(1 comment)

File Documentation/releases/coreboot-4.18-relnotes.md:

https://review.coreboot.org/c/coreboot/+/64843/comment/6b9536bd_6b7d9196 
PS1, Line 17:
> Sorry, I'm not really sure what is supposed to go into which notes here. […]

When you want to deprecate something, this should be announced 6 months before. So the earliest point for removing would be _after_ the 4.19 release and thus it should go to the 4.17 release notes.