[#827] Add README text on usage of iRODSSession instances

README.md

@@ -38,20 +38,61 @@ Uninstalling
 
     pip uninstall python-irodsclient
 
-Establishing a (secure) connection
-----------------------------------
+Establishing a connection
+-------------------------
 
-One way of starting a session is to pass iRODS credentials as keyword
-arguments:
+An `iRODSSession` instance is the interface object through which iRODS server
+APIs can be invoked. One can create the object using the constructor form directly,
+passing connection and authentication options within the call parameter list:
 
 ```python
 >>> from irods.session import iRODSSession
 >>> with iRODSSession(host='localhost', port=1247, user='bob', password='1234', zone='tempZone') as session:
-...      # workload
-...
->>>
+...     # Any number of operations using the 'session' variable can go here.
 ```
 
+Another way to create the session object, assuming one has already successfully
+set up a client environment via `iinit`, is by using the convenience function `make_session`:
+
+```python
+>>> from irods.helpers import make_session
+>>> session = make_session()
+```
+
+Once created, the `iRODSSession` instance can be managed from a choice between two
+possible patterns.  Firstly, one can allow references to the instance to persist as
+is natural for the application.  This allows Python interpreter's reference counting to
+let the object pass out of scope and destroy the underlying server connection(s) at the
+proper time.  This casual approach usually ends up being generally the most efficient one,
+as connections are expensive to create and destroy; furthermore, any single connection to
+the iRODS server can freely be employed for a number of disparate server interactions,
+one after the other.
+
+Alternatively, a context manager may be used.  This forces all connections to be
+provisionally cleared from the session object once a given block of code has
+executed:
+
+```python
+with make_session() as session:
+  my_user = session.users.get(session.username)
+  # We can have further usage of 'session' in this code block. At the end 
+  # of it, session.cleanup() is implicitly called to remove any idle connections.
+```
+
+Either way, the session instance remains available for further use afterward, until
+destructed.
+
+Of course, we should be mindful of how many still-connected `iRODSSession`
+objects are allowed to proliferate in an application, since having more client connections
+than the server can support database connections for can result in the spurious failure
+of any new attempts to connect.
+
+Basic authentication and security
+---------------------------------
+
+As seen in the `iRODSSession` constructor call in the previous section, iRODS credentials may be passed
+in as keyword arguments.
+
 If you're an administrator acting on behalf of another user:
 
 ```python