Add how to solve pipenv error to readme (#403)

* Add how to solve pipenv error to readme

* review comments

* tweak

* review comments

Author: ki4070ma

README.md

@@ -55,8 +55,12 @@ download and unarchive the source tarball (Appium-Python-Client-X.X.tar.gz).
     - It generates readable changelog
 - Setup
     - `pip install --user pipenv`
-    - `pipenv lock --clear`
-    - `pipenv install --dev --system`
+    - `python -m pipenv lock --clear`
+       - If you experience the below error, then refer [pypa/pipenv#187](https://github.com/pypa/pipenv/issues/187) to solve it.
+          ```
+          Locking Failed! unknown locale: UTF-8
+          ```
+    - `python -m pipenv install --dev --system`
     - `pre-commit install`
 
 ## Run tests