Readme.md: Update the Gitbook documentation section

We no longer use Gitbook for publishing the documentation.
Update the section to direct users to use the mailing list to
send feedback, and since we use GitHub Flavored Markdown, remove the
link to Gitbook.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Rebecca Cran <[email protected]>
Reviewed-by: Michael D Kinney <[email protected]>

Author: bexcran

Readme.md

@@ -30,12 +30,11 @@ documentation.
 
 # EDK II GitBook Documents
 
-These are the latest draft revisions published using [Gitbook](https://legacy.gitbook.com/).
+These are the latest draft revisions.
 The source content for these documents are in GIT repositories in the
 [Tianocore-docs](https://github.com/tianocore-docs) organization hosted by [GitHub](https://github.com).
-Feedback on these documents may be provided using the [Gitbook](https://www.gitbook.com) commenting feature
-available when reading the **HTML** versions of the documents. Document issues and feature requests can also
-be entered in [Tianocore Bugzilla](https://bugzilla.tianocore.org).
+Feedback on these documents may be provided via the mailing list.
+Document issues and feature requests can also be entered in [Tianocore Bugzilla](https://bugzilla.tianocore.org).
 
 * **_EDK II Build Specification_** \[
 [HTML   ](https://tianocore-docs.github.io/edk2-BuildSpecification/draft/),