prepare-font: add docs about XML preparation

docs/prepare-font.md

@@ -0,0 +1,23 @@
+How to prepare XML from TTF
+===========================
+
+1. To gather font metrics:
+   ```console
+   $ ttf2tfm filename.ttf -v filename.vpl
+   $ tftopl filename.tfm > filename.tpl
+   ```
+   That will generate `filename.tfm` (binary) and `filename.vpl`, `filename.tpl`
+   (text).
+
+   `ttf2tfm` and `tftopl` are parts of the standard TeX distribution (at least
+   TeX Live).
+2. May be helpful:
+   ```console
+   $ ttx filename.ttf
+   ```
+
+   That will generate `filename.ttx` (XML).
+
+   [ttx][]
+
+[ttx]: https://github.com/fonttools/fonttools