Advertise the fact that it is FAST (#8090)

I looked at the readme for this project over a year ago and it didn't say anything about improved performance, so I didn't bother to try it out. I don't really care about any of the features currently advertised.

I have learned that on my Kobo Aura HD, KOReader is much much faster than the built in reading software. I wish I would have been using it this whole time! (see https://www.mobileread.com/forums/showthread.php?p=4147051&posted=1#post4147051 for my test videos if you're interested)

README.md

@@ -28,6 +28,8 @@
 
 * **extensible**: via plugins
 
+* **fast**: on some older devices, it has been measured to have less than half the page-turn delay as the built in reading software.
+
 * **and much more**: look up words with StarDict dictionaries / Wikipedia, add your own online OPDS catalogs and RSS feeds, share ebooks with other KOReader devices wirelessly, online over-the-air software updates, an FTP client, an SSH server, …
 
 Please check the [wiki][link-wiki] to discover more features and to help us document them.