Update README.md

Add the pypi package name and example installation step.

At the moment users need to click into doc, then click installation to get the package name and install steps. Lets just add this most common of steps to the readme.

Author: mattseymour

README.md

@@ -9,6 +9,12 @@ require defining a schema or form. I wanted to create a simple validation
 library where validating a simple value does not require defining a form or a
 schema.
 
+```shell
+pip install validators
+```
+
+Then,
+
 ```python
 >>> import validators
 >>>