[SR-10149] Specify sphinx version

Resolve: https://bugs.swift.org/browse/SR-10149

Author: nafu

README.md

@@ -295,7 +295,7 @@ To read the compiler documentation, start by installing the
 [Sphinx](http://sphinx-doc.org) documentation generator tool by running the
 command:
 
-    easy_install -U Sphinx
+    easy_install -U "Sphinx < 2.0"
 
 Once complete, you can build the Swift documentation by changing directory into
 [docs](https://github.com/apple/swift/tree/master/docs) and typing `make`. This