Highlight remark about ease of implementation

Add the preprocessor recognised keyword "Note" to have Bikeshed put this
remark in a highlight box.

Signed-off-by: Joshua Lock <[email protected]>

Author: joshuagl

tuf-spec.md

@@ -19,12 +19,12 @@ Metadata Include: This version off, Abstract off
 Text Macro: VERSION 1.0.17
 </pre>
 
-We strive to make the specification easy to implement, so if you come across
-any inconsistencies or experience any difficulty, do let us know by sending an
-email to our [mailing
-list](https://groups.google.com/forum/?fromgroups#!forum/theupdateframework),
-or by reporting an issue in the [specification
-repo](https://github.com/theupdateframework/specification/issues).
+Note: We strive to make the specification easy to implement, so if you come
+across any inconsistencies or experience any difficulty, do let us know by
+sending an email to our [mailing list](
+  https://groups.google.com/forum/?fromgroups#!forum/theupdateframework),
+or by reporting an issue in the [specification repo](
+  https://github.com/theupdateframework/specification/issues).
 
 
 # Introduction # {#introduction}