README.md: Mention host requirements for building the docs

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>

README.md

@@ -32,6 +32,11 @@ by simply running:
 make
 ```
 
+For the above command to succeed, the host needs to provide `sphinx-build` and
+the following sphinx extensions:
+* sphinx-tabs (supports tabbed content in the document)
+* sphinxcontrib.plantuml (supports plantuml content)
+
 ## Contributing
 
 See the `CONTRIBUTING.md` file.