Add note about the Nokogiri env var

Author: eregon

lib/mri/mkmf.rb

@@ -51,6 +51,7 @@
 if defined?(::TruffleRuby)
   # Always use the system libxml/libxslt for Nokogiri on TruffleRuby.  This is
   # currently required as TruffleRuby cannot yet link to static libraries.
+  # See https://github.com/sparklemotion/nokogiri/pull/2085 when solving this.
   ENV['NOKOGIRI_USE_SYSTEM_LIBRARIES'] = 'true'
 end