Merge pull request #142 from readthedocs/sphinx-pinning-policy

Add clarification on pinning policy

Author: astrojuanlu

build-errors-docutils-0-18.rst

@@ -48,6 +48,12 @@ with Sphinx versions 1.x and 2.x, however these versions do not specify a upper
 limit to the version of docutils installed, and so will install the latest
 release by default.
 
+The reason so many projects use an old version of Sphinx
+is that, :ref:`before October 2020, Read the Docs had a more strict pinning
+policy <readthedocs:builds:External dependencies>`
+and Sphinx 1.8 was the default version.
+New projects are not affected.
+
 Fixing the error
 ----------------