add the basic usage documentation

README.md

@@ -12,7 +12,85 @@ A pure python library for building and parsing message as part of the AS2 messag
 * Parsing a received MIME data and identifying if it as a Message or MDN. 
 * Decompress, Decrypt and Verify Signature of the received payload.
 * Verify Signature of the received MDN and extract original message status. 
-
+
+
+## Basic Usage
+
+Let us take a look at how we can use this library for building and parsing of AS2 Messages. 
+
+### Setup
+
+* First we would need to setup an organization and a partner
+```python
+from pyas2lib.as2 import Organization, Partner
+
+my_org = Organization(
+    as2_name='my_unique_id',  # Unique AS2 Id for this organization
+    sign_key=b'signature_key_bytes',  # PEM/DER encoded private key for signature
+    sign_key_pass='password',  # Password private key for signature
+    decrypt_key=b'decrypt_key_bytes',  # PEM/DER encoded private key for decryption
+    decrypt_key_pass='password'  # Password private key for decryption
+)
+
+a_partner = Partner(
+    as2_name='partner_unique_id',  # Unique AS2 Id of your partner
+    sign=True,  # Set to true for signing the message
+    verify_cert=b'verify_cert_bytes',  # PEM/DER encoded certificate for verifying partner signatures
+    encrypt=True,  # Set to true for encrypting the message
+    encrypt_cert=b'encrypt_cert_bytes',  # PEM/DER encoded certificate for encrypting messages
+    mdn_mode='SYNC',  # Expect to receive synchronous MDNs from this partner
+    mdn_digest_alg='sha256'  # Expect signed MDNs to be returned by this partner
+)
+
+``` 
+
+### Sending a message to your partner
+
+* The partner is now setup we can build and AS2 message
+```python
+from pyas2lib.as2 import Message
+
+msg = Message(sender=my_org, receiver=a_partner)
+msg.build(b'data_to_transmit')
+
+```
+* The message is built and now `msg.content` holds the message body and `message.header` dictionary holds the message headers. These need to be passed to any http library for HTTP POSTing to the partner.
+* We expect synchronous MDNs so we need to process the response to our HTTP POST
+```python
+from pyas2lib.as2 import Mdn
+
+msg_mdn = Mdn()  # Initialize an Mdn object
+
+# Call the parse method with the HTTP response headers + content and a function that returns the related `pyas2lib.as2.Messsage` object.
+status, detailed_status = msg_mdn.parse(b'response_data_with_headers', find_message_func)
+```
+* We parse the response mdn to get the status and detailed status of the message that was transmitted.
+
+### Receiving a message from your partner
+
+* We need to setup and HTTP server with an endpoint for receiving POST requests fro your partner.
+* When a requests is received we need to first check if this is an Async MDN
+```python
+from pyas2lib.as2 import Mdn
+
+msg_mdn = Mdn()  # Initialize an Mdn object
+# Call the parse method with the HTTP request headers + content and a function the returns the related `pyas2lib.as2.Messsage` object.
+status, detailed_status = msg_mdn.parse(request_body, find_message_fumc)
+```
+* If this is an Async MDN it will return the status of the original message.
+* In case the request is not an MDN then `pyas2lib.exceptions.MDNNotFound` is raised, which needs to be catched and parse the request as a message.
+```python
+from pyas2lib.as2 import Message
+
+msg = Message()
+# Call the parse method with the HTTP request headers + content, a function to return the the related `pyas2lib.as2.Organization` object, a function to return the `pyas2lib.as2.Partner` object and a function to check for duplicates.
+status, exception, mdn = msg.parse(
+    request_body, find_organization, find_partner, check_duplicate_msg)
+```
+* The parse function returns a 3 element tuple; the status of parsing, exception if any raised during parsing and an `pyas2lib.as2.Mdn` object for the message.
+* If the `mdn.mdn_mode` is `SYNC` then the `mdn.content` and `mdn.header` must be returned in the response.
+* If the `mdn.mdn_mode` is `ASYNC` then the mdn must be saved for later processing.  
+
 ## Contribute
 
 1. Check for open issues or open a fresh issue to start a discussion around a feature idea or a bug.