[FAB-8440] Create initial configtxgen command md

Ref: FAB-8179
[ci-skip]

All CLI functions are to be documented as part of v1.1.  This CR makes
an initial pass to document the basic functionality of the configtxgen
command.  A subsequent CR to follow merging any of the relevant content
from configtxgen.rst.

Change-Id: I26b98d03f52c72007dafcc6c8d1a878bdb5d2c7c
Signed-off-by: Jason Yellick <jyellick@us.ibm.com>

docs/source/commands/configtxgen.md

@@ -0,0 +1,95 @@
+configtxgen
+=============
+
+## Description
+
+The `configtxgen` command allows users to create and inspect channel config
+related artifacts.  The content of the generated artifacts is dictated by the
+contents of `configtx.yaml`.
+
+## Syntax
+
+The `configtxgen` tool has no sub-commands, but supports flags which can be set
+to accomplish a number of tasks.
+
+```
+Usage of configtxgen:
+  -asOrg string
+    	Performs the config generation as a particular organization (by name), only including values in the write set that org (likely) has privilege to set
+  -channelID string
+    	The channel ID to use in the configtx (default "testchainid")
+  -inspectBlock string
+    	Prints the configuration contained in the block at the specified path
+  -inspectChannelCreateTx string
+    	Prints the configuration contained in the transaction at the specified path
+  -outputAnchorPeersUpdate string
+    	Creates an config update to update an anchor peer (works only with the default channel creation, and only for the first update)
+  -outputBlock string
+    	The path to write the genesis block to (if set)
+  -outputCreateChannelTx string
+    	The path to write a channel creation configtx to (if set)
+  -printOrg string
+    	Prints the definition of an organization as JSON. (useful for adding an org to a channel manually)
+  -profile string
+    	The profile from configtx.yaml to use for generation. (default "SampleInsecureSolo")
+  -version
+    	Show version information
+```
+
+## Usage
+
+### Output a genesis block
+
+Write a genesis block to `genesis_block.pb` for channel `orderer-system-channel`
+for profile `SampleSingleMSPSolo`.
+
+```
+configtxgen -outputBlock genesis_block.pb -profile SampleSingleMSPSolo -channelID orderer-system-channel
+```
+
+### Output a channel creation tx
+
+Write a channel creation transaction to `create_chan_tx.pb` for profile
+`SampleSingleMSPChannel`.
+
+```
+configtxgen -outputCreateChannelTx create_chan_tx.pb -profile SampleSingleMSPChannel -channelID application-channel-1
+```
+
+### Inspect a genesis block
+
+Print the contents of a genesis block named `genesis_block.pb` to the screen as
+JSON.
+
+```
+configtxgen -inspectBlock genesis_block.pb
+```
+
+### Inspect a channel creation tx
+
+Print the contents of a channel creation tx named `create_chan_tx.pb` to the
+screen as JSON.
+
+```
+configtxgen -inspectChannelCreateTx create_chan_tx.pb
+```
+
+### Print an organization definition
+
+Construct an organization definition based on the parameters such as MSPDir
+from `configtx.yaml` and print it as JSON to the screen. (This output is useful
+for channel reconfiguration workflows, such as adding a member).
+
+```
+configtxgen -printOrg Org1
+```
+
+### Output anchor peer tx
+
+Output a configuration update transaction to `anchor_peer_tx.pb` which sets the
+anchor peers for organization Org1 as defined in profile SampleSingleMSPChannel
+based on `configtx.yaml`.
+
+```
+configtxgen -outputAnchorPeersUpdate anchor_peer_tx.pb -profile SampleSingleMSPChannel -asOrg Org1
+```