Add section on _meta (#201)

* Add section on _meta

* more fields to the block

pages/en/querying/graphql-api.mdx

@@ -316,3 +316,33 @@ GraphQL schemas generally define root types for `queries`, `subscriptions` and `
 All GraphQL types with `@entity` directives in your schema will be treated as entities and must have an `ID` field.
 
 > **Note:** Currently, all types in your schema must have an `@entity` directive. In the future, we will treat types without an `@entity` directive as value objects, but this is not yet supported.
+
+### Subgraph Metadata
+
+All subgraphs have an auto-generated `_Meta_` object, which provides access to subgraph metadata. This can be queried as follows:
+
+```graphQL
+{
+  _meta(block: { number: 123987 }) {
+    block {
+      number
+      hash
+      timestamp
+    }
+    deployment
+    hasIndexingErrors
+  }
+}
+```
+
+If a block is provided, the metadata is as of that block, if not the latest indexed block is used. If provided, the block must be after the subgraph's start block, and less than or equal to the most recently indexed block.
+
+`deployment` is a unique ID, corresponding to the IPFS CID of the `subgraph.yaml` file.
+
+`block` provides information about the latest block (taking into account any block constraints passed to _meta):
+- hash: the hash of the block
+- number: the block number
+- timestamp: the timestamp of the block, if available (this is currently only available for subgraphs indexing EVM networks)
+
+`hasIndexingErrors` is a boolean identifying whether the subgraph encountered indexing errors at some past block
+