add contract metadata docs

docs/contract_metadata.md

+Contract Metadata
+=================
+
+As compilation succeed Remix create a JSON file for each compiled contract.
+These JSON files contains several metadata
+
+Library Deployment
+------------------
+
+By default Remix automatically deploy needed libraries.
+
+`linkReferences` contains a map representing libraries which depend on the current contract. 
+Values are addresses of libraries used for linking the contract.
+
+`autoDeployLib` defines if the libraries should be auto deployed by Remix or if the contract should be linked with libraries described in `linkReferences`
+
+```
+{
+	"linkReferences": {
+		"browser/Untitled.sol": {
+			"lib": "0x8c1ed7e19abaa9f23c476da86dc1577f1ef401f5"
+		}
+	},
+	"autoDeployLib": false
+}
+```

docs/index.rst

@@ -43,6 +43,7 @@ Userful links:
 
    run_tab
    udapp
+   contract_metadata
 
 .. toctree::
    :maxdepth: 2