doc: add sample code to README

README.md

@@ -1 +1,77 @@
 # pyx12-lib
+
+---
+
+## Quick Example
+
+### Rendering
+* Define the grammar for the segment.
+```python
+from pyx12lib.core.grammar import Element, BaseSegment, segment, element
+
+
+class StSegment(BaseSegment):
+    segment_id = 'ST'
+    usage = segment.USAGE_MANDATORY
+    max_use = 1
+    elements = (
+        Element(
+            reference_designator='ST01',
+            name='Transaction Set Identifier Code',
+            usage=element.USAGE_MANDATORY,
+            element_type=element.ELEMENT_TYPE_ID,
+            minimum=3,
+            maximum=3,
+        ),
+        Element(
+            reference_designator='ST02',
+            name='Transaction Set Control Number',
+            usage=element.USAGE_MANDATORY,
+            element_type=element.ELEMENT_TYPE_STRING,
+            minimum=4,
+            maximum=9,
+        ),
+    )
+```
+* Render the segment from its grammar.
+```python
+from pyx12lib.common.interchange.grammar import StSegment
+from pyx12lib.core.renderer import SegmentRenderer
+
+
+class StRenderer(SegmentRenderer):
+    grammar = StSegment
+
+    element_value_getters =  {
+        'ST01': lambda ele, data, stat: '997',
+        'ST02': lambda ele, data, stat: '{:04d}'.format(data.transaction_set_no),
+    }
+```
+* Advanced definition for element value getters
+```python
+from pyx12lib.common.interchange.grammar import StSegment
+from pyx12lib.core.renderer import SegmentRenderer
+
+
+class StRenderer(SegmentRenderer):
+    grammar = StSegment
+
+    @property
+    def element_value_getters(self):
+        return {
+            'ST01': lambda ele, data, stat: '997',
+            'ST02': self.st02,
+        }
+
+    @staticmethod
+    def st02(ele, data, stat):
+        assert ele is StSegment.elements[1]  # the grammar for the element.
+        assert stat['ST01'] is '997'  # stat consists of rendered data so far.
+        return '{:04d}'.format(data.transaction_set_no)  # return value should always be strings
+```
+
+---
+## Test
+```bash
+python -m unittest discover
+```