docs(follower): add follower node documentation.

docs/follower_node.md

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+# Follower Node
+
+When started with `"EnableFollowMode": true`, algod starts with a special
+property which allows it to be paused. This allows software to be written
+which runs synchronously with a node. If you need account balances at each
+round, or at a particular round, this is the only way to do it. On start
+the node will be paused, the REST API is used to resume and select a new
+round.
+
+It also allows fetching a `Ledger State Delta` object for recent rounds.
+This object is not designed for external users, but may be useful for
+advanced applications if you're willing to figure things out. It contains
+complete information for transitioning a database to the next round,
+including new account balances, changes to application and asset states,
+and new box information. Such information was previously unavailable to
+application developers.
+
+## Configuration
+
+Several options in the `config.json` file are needed:
+
+| property | description |
+| EnableFollowMode | When set to `true` the node starts as a network follower. | 
+| MaxAcctLookback | The number of additional `Ledger State Delta` objects available. |
+| CatchupParallelBlocks | This is useful for performance tuning. |
+
+## Usage
+
+New public endpoints are available to control the node:
+* `GET /v2/ledger/sync` - fetch the current minimum sync round.
+* `DELETE /v2/ledger/sync` - resume normal catchup by deleting the sync round.
+* `POST /v2/ledger/sync/{round}` - set the sync round.
+
+The `Ledger State Delta` object is not designed for external consumption,
+but may still be useful for advanced applications:
+* `GET /v2/deltas/{round}` - Fetch the raw Ledger State Delta, optionally provide `format=msgp` for the internal msgp encoded object.
+
+## Performance
+
+Increasing `MaxAcctLookback` and `CatchupParallelBlocks` is helpful for
+increasing performance. We have found `64` to be a good value which allows
+algod to at near full speed while a fast synchronous application processes
+`Ledger State Delta` objects.
+
+## Restrictions
+
+The follower node was stripped of all functionality not directly related to
+assisting with data-gathering capabilities. Since it is designed to run
+alongside another application, it was made as lightweight as possible.
+Other restrictions relate to the fact that this node is designed to be
+paused. So there are no guarantees that it's internal state matches the
+current round of consensus.
+
+In particular, the follower node cannot participate in consensus and any
+attempt to register a key will be rejected.