Annotate text fields with supported formatting

Is your feature request related to a problem? Please describe. When making a post on Bookwyrm (a review, adding a book to my shelf, etc) I'd like to use some formatting, but I don't know what formatting is supported.

Describe the solution you'd like Add a little annotation under the text box describing what formatting is supported, like "Bookwyrm supports CommonMark" or something. (I don't know if you actually support CommonMark, but clearly some kind of markdown is supported.) If it needs a little more explanation, you could have a click to expand section or a link to a formatting page.

Describe alternatives you've considered I haven't really considered alternatives to this, because it's a standard and user friendly way to inform users of their formatting options.

Additional context None.