Add indexer guide

docs/concepts/indexer.md

@@ -0,0 +1,18 @@
+---
+id: indexer
+title: NEAR Indexer
+sidebar_label: Indexer
+---
+
+> NEAR Indexer is a library included with [nearcore](https://github.com/near/nearcore) that allows you to run a node on the network which listens for targeted information on the blockchain. The [NEAR Indexer Framework](https://github.com/near/nearcore/tree/master/chain/indexer) provides the logic of polling a node for blocks produced in a network then aggregating and streaming these blocks to a listener. 
+
+To better understand this, blockchain data is optimized for serialized writes, one block at a time, as the chain is being created. Querying the blockchain for data about a specific block or account is a fairly stragightforward or "narrow" query. However, querying data across many blocks can be cumbersome because we have to aggregate results from multiple single-block queries. We can consider these "wide" queries.
+
+- An indexer listens to the stream of data as it's being written on chain and can then be immediately filtered and processed to detect interesting events or patterns.
+- This stream of data can then be written to a permanent database for later data analysis using a convenient query language like SQL.
+
+- One example that highlights the need for a "wide query" is when you use a seed phrase to recover one or more accounts. Since a seed phrase essentially represents a signing key pair, the recovery is for **all** accounts that share the associated public key. Therefore, when a seed phrase is used to recover an account via [NEAR Wallet](https://wallet.near.org/), the query requires that **all accounts** with a matching public key are found and recovered.
+
+- The easiest way to achieve a "wide query" is to utilize a database that has been filled by an indexing service. For this purpose we’ve built the [NEAR Indexer for wallet](https://github.com/near/near-indexer-for-wallet) which listens to all actions on chain that might create or delete [access keys](/docs/concepts/account#access-keys) and stores them into a relational database for easier querying of accounts.
+
+[ **[Click Here](/docs/tools/near-indexer)** ] For a walkthrough on creating an indexer.