Be more specific about what version of luaotfload is needed to build …

…docs.

README.md

@@ -19,8 +19,8 @@ You need a recent and working [TeXLive](https://www.tug.org/texlive/), on top of
 **Note:** Due to a bug in the current release (v2.7-fix-3) of `luaotfload` the documentation cannot be built against TeXLive 2016 (see [#1188](https://github.com/gregorio-project/gregorio/issues/1188)).  As a result, building from a clone of the repository will fail against that version.  A work around for this bug exists ([9fe9139](https://github.com/henryso/gregorio/commit/9fe9139178dce6adebffbae65016800748450b61), but using it breaks compilation under TeXLive 2015 and the bug has been fixed in the latest master for `luaotfload`.  The result is that users should do one of the following:
 
 - Install from the distribution tar ball (recommended for those installing a stable release).
-- Switch to TeXLive 2015 to build the docs (this is what the developers do when creating the distribution tar ball).
-- Upgrade your `luaotfload` to the lastest master.
+- Switch to TeXLive 2015 to build the docs.
+- Upgrade your `luaotfload` to a more recent version (v2.8-rc1 or later).
 
 ## Documentation