Update README.md

Author: rupert

README.md

@@ -4,10 +4,14 @@
 
 > [Black](https://github.com/ambv/black) plugin for the [Python Language Server](https://github.com/palantir/python-language-server).
 
+In the same `virtualenv` as `python-language-server`:
+
 ```shell
 pip3 install pyls-black
 ```
 
+To avoid unexpected results you should make sure `yapf` and `autopep8` are not installed.
+
 * `pyls-black` can either format an entire file or just the selected text.
 * The code will only be formatted if it is syntactically valid Python.
 * Text selections are treated as if they were a separate Python file.