Rename fonts for epub renderer

This fixes rendering issues on the Kobo epub renderer which is used
when Kobo is not reading a kepub file, but a regular epub file.

It seems that the fallback fonts are loaded differently when loading
a regular .epub file, and fonts ending in -Roman.ttf are not loaded
correctly. This is resolved when the usual naming policy is applied.

Only NV Charter and NV Palatium used the incorrect file names.
Only users reading epub files on Kobo devices would have run into
this issue, but this is a silly one.

Also added addendum about kepubify to the README related to the
different renderers.

README.md

@@ -74,6 +74,10 @@ To reboot, long-press the power button until your Kobo says it has been shut dow
 * NV Charter is available under the original Bitstream license, which is included in the respective archive.
 * NV Palatium is available under the OFL license, and is available under the same license as Domitian is.
 
+### Is there anything else I should do?
+
+If you are manually transferring books to your Kobo devices, you should consider converting `epub` files to `kepub` files. To do this, you can use [kepubify](https://pgaskin.net/kepubify/) or use [Calibre](https://calibre-ebook.com/). This ensures that your Kobo device will use a superior and faster book renderer. This renderer also gives you broader font compatibility.
+
 ### What is your favorite font from the collection?
 
 * For simple books and basic readability, I personally prefer NV Charter or NV Jost.