Add note about lrelease

Author: CDrummond

README

@@ -808,6 +808,10 @@ need to set CMAKE_PREFIX_PATH e.g.
 
     export CMAKE_PREFIX_PATH=/usr/local/Cellar/qt5/5.9.1/lib/cmake/:${CMAKE_PREFIX_PATH}
 
+Ensure 'lrelease' (required to build Cantata translations) is in PATH, e.g.
+
+    export PATH=/usr/local/Cellar/qt5/5.9.1/bin/:${PATH}
+
 
 Create Installer
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