Clarify make commands for building docs

Author: hinerm

doc/Development.md

@@ -13,10 +13,18 @@ make test
 
 ## Building the reference documentation
 
+The documentation has its own `Makefile` in the `/docs` directory. From there:
+
 ```
 make html
 ```
 
+Alternatively, from the project root:
+
+```
+make docs
+```
+
 Results are generated to `doc/_build/html`.
 Production documentation is available online at
 [https://pyimagej.readthedocs.io/](https://pyimagej.readthedocs.io/).