Add light sync command

- Only syncs block headers (excludes block bodies, transactions, receipts, and logs)
- Modifies validation window to include the most recent block
- Isolates validation window to the variable defined in the cmd directory (blocks
  have a separate variable defined in the block_repository for determining when
  to set a block as final)

README.md

@@ -54,9 +54,10 @@ Vulcanize DB is a set of tools that make it easier for developers to write appli
 - See `environments/infura.toml` to configure commands to run against infura, if a local node is unavailable
 
 ## Start syncing with postgres
-Syncs VulcanizeDB with the configured Ethereum node.
-1. Start node (**if fast syncing wait for initial sync to finish**)
-1. In a separate terminal start vulcanize_db
+Syncs VulcanizeDB with the configured Ethereum node, populating blocks, transactions, receipts, and logs.
+This command is useful when you want to maintain a broad cache of what's happening on the blockchain.
+1. Start Ethereum node (**if fast syncing your Ethereum node, wait for initial sync to finish**)
+1. In a separate terminal start VulcanizeDB:
     - `./vulcanizedb sync --config <config.toml> --starting-block-number <block-number>`
 
 ## Alternatively, sync from Geth's underlying LevelDB
@@ -69,6 +70,13 @@ Sync VulcanizeDB from the LevelDB underlying a Geth node.
     - `--ending-block-number`/`-e`: block number to sync to
     - `--all`/`-a`: sync all missing blocks
 
+## Alternatively, sync in "light" mode
+Syncs VulcanizeDB with the configured Ethereum node, populating only block headers.
+This command is useful when you want a minimal baseline from which to track targeted data on the blockchain (e.g. individual smart contract storage values).
+1. Start Ethereum node
+2. In a separate terminal start VulcanizeDB:
+    - `./vulcanizedb lightSync --config <config.toml> --starting-block-number <block-number>`
+
 ## Start full environment in docker by single command
 
 ### Geth Rinkeby