Add full example on DataFrame usage & access (#100)

Add example of Python vs Clojure doing the same thing
  - import dependencies
  - create DataFrame
  - access its rows

To clarify how the `get-item` and `get-attr` work together.

After spending long time to figure out how the use the attributes vs
items access in Clojure to access DataFrame rows and columns, I thought
it would be useful to document full example how to do it Python &
Clojure side by side.

Author: madis

docs/Usage.md

@@ -8,6 +8,15 @@ item access is optional and happens via the `__getitem__` and `__setitem__` attr
 This is important to realize in that the code below doesn't look like python because we are
 referencing the item and attribute systems by name and not via '.' or '[]'.
 
+This would result in the following analogous code (full example [further on](#dataframe-access-full-example)):
+
+```python
+table.loc[row_date]
+```
+
+```clojure
+(get-item (get-attr table :loc) row-date)
+```
 
 ### Installation
 
@@ -216,6 +225,31 @@ user> (att-type-map ones-ary)
 }
 ```
 
+### DataFrame access full example
+
+Here's how to create Pandas DataFrame and accessing its rows via `loc` in both Python and Clojure:
+
+```python
+# Python
+import numpy as np
+import pandas as pan
+
+dates = pan.date_range("1/1/2000", periods=8)
+table = pan.DataFrame(np.random.randn(8, 4), index=dates, columns=["A", "B", "C", "D"])
+row_date = pan.date_range(start="2000-01-01", end="2000-01-01")
+table.loc[row_date]
+```
+
+```clojure
+; Clojure
+(require-python '[numpy :as np])
+(require-python '[pandas :as pan])
+
+(def dates (pan/date_range "1/1/2000" :periods 8))
+(def table (pan/DataFrame (call-attr np/random :randn 8 4) :index dates :columns ["A" "B" "C" "D"]))
+(def row-date (pan/date_range :start "2000-01-01" :end "2000-01-01"))
+(get-item (get-attr table :loc) row-date)
+```
 
 ### Errors