Fix ARM build on readthedocs.org

By default readthedocs.org uses Sphinx 1.8.5, but MR !4563 has
introduced depedency on ReferenceRole class which is available only in
Sphinx 2.0.0.

Path to doc/arm/requirements.txt needs to be configured in
readthedocs.org.

diff --git a/doc/arm/requirements.txt b/doc/arm/requirements.txt
new file mode 100644 (file)
index 0000000..bf5cf66
--- /dev/null
+++ b/doc/arm/requirements.txt
@@ -0,0 +1 @@
+Sphinx>=2