Update READAME.md

Expand the note about external library integration, the old text implied
that HarfBuzz produces different results on different OSs or targets,
which is not the case.

Also expand the note about Arabic fallback shaper (the new text is
copied from harfruzz README.md).

README.md

@@ -28,11 +28,10 @@ are not implemented yet or cannot be implemented at all.
 - Malformed fonts will cause an error. HarfBuzz uses fallback/dummy shaper in this case.
 - No font size property. Shaping is always using UnitsPerEm. You should scale the result manually.
 - Most of the TrueType and Unicode handling code was moved into separate crates.
-- rustybuzz doesn't interact with any system libraries and must produce exactly the same
-  results on all OS'es and targets.
+- rustybuzz doesn't provide any integration with external libraries, so no
+  FreeType, CoreText, or Uniscribe/DirectWrite font-loading integration, and no ICU, or GLib Unicode-functions integration, as well as no `graphite2` library support.
 - `mort` table is not supported, since it's deprecated by Apple.
-- No Arabic fallback shaper, since it requires subsetting.
-- No `graphite` library support.
+- No Arabic fallback shaper. This requires the ability to build lookups on the fly. In HarfBuzz (C++) this requires serialization code that is associated with subsetting.
 - `avar2` as well as other parts of the boring-expansion-spec are not supported yet.
 
 ## Performance