Use 90. prefix for back-matter.md

Merges manubot/rootstock#222
Closes manubot/manubot#106

Enables creating two-digit prefixed content markdown files that
will sort to come after back-matter. While back matter is intended
to be the final markdown file, some users may want to place content
after it.

USAGE.md

@@ -4,7 +4,8 @@ This repository uses [Manubot](https://manubot.org) to automatically produce a m
 
 ## Manubot markdown
 
-Manuscript text should be written in markdown files, which should be located in [`content`](content) with a digit prefix for ordering (e.g. `01.`, `02.`, etc.) and a `.md` extension.
+Manuscript text should be written in markdown files located in the [`content`](content) directory.
+Markdown files are identified by their `.md` extension and ordered according to their two-digit prefix (e.g. `01.`, `02.`, … `99.`).
 
 For basic formatting, check out the [CommonMark Help](https://commonmark.org/help/) page for an introduction to the formatting options provided by standard markdown.
 In addition, Manubot supports an extended version of markdown, tailored for scholarly writing, which includes [Pandoc's Markdown](https://pandoc.org/MANUAL.html#pandocs-markdown) and the extensions discussed below.