[FAB-6810] Document SCC Plugins

Change-Id: I8c97c862e84d095599437c759d28bcf032192d80
Signed-off-by: Divyank Katira <Divyank.Katira@securekey.com>

Author: d1vyank

docs/source/index.rst

@@ -55,6 +55,7 @@ Business:
    chaincode
    chaincode4ade
    chaincode4noah
+   systemchaincode
    videos
 
 .. toctree::

docs/source/systemchaincode.rst

@@ -0,0 +1,67 @@
+System Chaincode Plugins
+========================
+
+System chaincodes are specialized chaincodes that run as part of the peer process
+as opposed to user chaincodes that run in separate docker containers. As
+such they have more access to resources in the peer and can be used for
+implementing features that are difficult or impossible to be implemented through
+user chaincodes. Examples of System Chaincodes are ESCC (Endorser System Chaincode)
+for endorsing proposals, QSCC (Query System Chaincode) for ledger and other
+fabric related queries and VSCC (Validation System Chaincode) for validating
+a transaction at commit time respectively.
+
+Unlike a user chaincode, a system chaincode is not installed and instantiated
+using proposals from SDKs or CLI. It is registered and deployed by the peer at start-up.
+
+System chaincodes can be linked to a peer in two ways: statically and dynamically
+using Go plugins. This tutorial will outline how to develop and load system chaincodes
+as plugins.
+
+Developing Plugins
+------------------
+
+A system chaincode is a program written in `Go <https://golang.org>`_ and loaded
+using the Go `plugin <https://golang.org/pkg/plugin>`_ package.
+
+A plugin includes a main package with exported symbols and is built with the command
+``go build -buildmode=plugin``.
+
+Every system chaincode must implement the `Chaincode Interface <http://godoc.org/github.com/hyperledger/fabric/core/chaincode/shim#Chaincode>`_
+and export a constructor method that matches the signature ``func New() shim.Chaincode`` in the main package.
+An example can be found in the repository at ``examples/plugin/scc``.
+
+Existing chaincodes such as the QSCC can also serve as templates for certain
+features - such as access control - that are typically implemented through system chaincodes.
+The existing system chaincodes also serve as a reference for best-practices on
+things like logging and testing.
+
+.. note:: On imported packages: the Go standard library requires that a plugin must
+          include the same version of imported packages as the host application (fabric, in this case)
+
+Configuring Plugins
+-------------------
+
+Plugins are configured in the ``chaincode.systemPlugin`` section in *core.yaml*:
+
+.. code-block:: bash
+
+  chaincode:
+    systemPlugins:
+      - enabled: true
+        name: mysyscc
+        path: /opt/lib/syscc.so
+        invokableExternal: true
+        invokableCC2CC: true
+
+
+A system chaincode must also be whitelisted in the ``chaincode.system`` section in *core.yaml*:
+
+.. code-block:: bash
+
+  chaincode:
+    system:
+      mysyscc: enable
+
+
+.. Licensed under Creative Commons Attribution 4.0 International License
+   https://creativecommons.org/licenses/by/4.0/