CSS markup allows you to assign visual styles (or semantic CSS
class names) to a block of text (paragraph) or to pieces of text.
@@ is used to start a styled area. The @@ must be immediately
followed by either a CSS class name (without the dot) or with
CSS instructions without any whitespaces.
The following text (after the  the class name and a space) will
then be assigned the class until a (possible) next  without
attached classname or style definition.

If the @@ occurs at the start of a paragraph it will enclose it
in a <div> with the according style assignment, otherwise (in the
text) a @@ will become a <span>.

See also the explanation and examples in this plugins` comments.
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You cannot modify the README.plugins file, but anyhow any ideas or suggestion should as usually get filed on BugReports, UserSuggestions or even better the README.plugins.Discussion.