Simplify the installation method since we released on PyPI already

Author: rayluo

README.md

@@ -27,7 +27,7 @@ as applications written using a preview version of library may no longer work.
 
 1. If you haven't already, [install and/or upgrade the pip](https://pip.pypa.io/en/stable/installing/)
    of your Python environment to a recent version. We tested with pip 18.1.
-2. For now, you can install from our latest dev branch, by `pip install git+https://github.com/AzureAD/microsoft-authentication-library-for-python.git@dev`
+2. As usual, just run `pip install msal`.
 
 ## Usage
 
@@ -45,7 +45,7 @@ Acquiring tokens with MSAL Python need to follow this 3-step pattern.
    from msal import PublicClientApplication
    app = PublicClientApplication("your_client_id", authority="...")
    ```
-   
+
    Later, each time you would want an access token, you start by:
    ```python
    result = None  # It is just an initial value. Please follow instructions below.