Note about feature flags for uefi book

[stillinbeta]

It took me most of an evening to figure this out, so I thought I'd share it!

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See the Rewriting History guide for help.
• Update the changelog (if necessary)

[phip1611]

Sorry for the inconvenience you experienced. Great catch! I left some remarks.

@nicholasbishop I think we should expose all features except for unstable as default feature.

[nicholasbishop]

@nicholasbishop I think we should expose all features except for unstable as default feature.

Sounds good to me.

[nicholasbishop]

Merged, thanks for taking the time to document this :)

Filed a followup ticket to set default features.