Update and rename environmental_variables.rst to environment_variables.rst

Made review requests per @hardym.

docs/guide/environment_variables.rst

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+Environment Variables
+=======================
+
+Best practice holds that variables should be passed to graphs via environment variables.
+Doing this is important for keeping sensitive data out of the code - such as an
+API token or username and password used to access a database. Not only is this
+approach more secure, it also makes graphs more flexible by allowing adjustments
+for a variety of environments and contexts. Importantly, environment variables
+are also the means by-which arguments can be passed to graphs.
+
+
+Passing / Setting Environment Variables
+::::::::::::::::::::::::::::::::::::::::::::
+
+Setting environment variables for your graphs to use can be done in a variety of ways and which one used can vary
+based-upon context. Perhaps the most immediate and simple way to set/override a variable for a given graph is 
+simply to use the optional ``--env`` argument when running bonobo from the shell (bash, command prompt, etc). 
+``--env`` (or ``-e`` for short) should then be followed by the variable name and value using the
+syntax `VAR_NAME=VAR_VALUE`. Multiple environment variables can be passed by using multiple ``--env`` / ``-e`` flags. Additionally, in bash you can also set environment variables by listing those you wish to set before the `bonobo run` command with space separating the key-value pairs (i.e. `FIZZ=buzz bonobo run ...` or `FIZZ=buzz FOO=bar bonobo run ...`). 
+
+The Examples below demonstrate setting one or multiple variables using both of these methods:
+
+.. code-block:: bash
+
+    # Using one environment variable via --env flag:
+    bonobo run csvsanitizer --env SECRET_TOKEN=secret123
+
+    # Using multiple environment variables via -e (env) flag:
+    bonobo run csvsanitizer -e SRC_FILE=inventory.txt -e DST_FILE=inventory_processed.csv
+    
+    # Using one environment variable in bash (*bash only):
+    SECRET_TOKEN=secret123 bonobo run csvsanitizer
+
+    # Using multiple environment variables in bash (*bash only):
+    SRC_FILE=inventory.txt DST_FILE=inventory_processed.csv bonobo run csvsanitizer
+    
+*Though not-yet implemented, the bonobo roadmap includes implementing environment / .env files as well.*
+
+Accessing Environment Variables from within the Graph Context
+:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
+
+Environment variables, whether global or only for the scope of the graph,
+can be can be accessed using any of the normal means. It is important to note
+that whether set globally for the system or just for the graph context,
+environment variables are accessed by bonobo in the same way. In the example
+below the database user and password are accessed via the ``os`` module's ``getenv``
+function and used to get data from the database.
+
+.. code-block:: python
+
+    import os
+
+    from bonobo import Graph, run
+
+
+    def extract():
+        database_user = os.getenv('DB_USER')
+        database_password = os.getenv('DB_PASS')
+        # ...
+        # (connect to database using database_user and database_password)
+        # (get data from database)
+        # ...
+
+        return database_data
+
+
+    def load(database_data: dict):
+        for k, v in database_data.items():
+            print('{key} = {value}'.format(key=k, value=v))
+
+
+    graph = Graph(extract, load)
+
+    if __name__ == '__main__':
+        run(graph)