Add a requirements file

Change-Id: I892ce76b857a76c4a76a72b2386dba71cd885eb9
diff --git a/requirements.txt b/requirements.txt
new file mode 100644
index 0000000..c452ba1
--- /dev/null
+++ b/requirements.txt
@@ -0,0 +1 @@
+Sphinx~=4.0