add json adaptor docs

docs/importer/adaptors/json.md

+# JSON adaptor
+
+The json adaptor reads data from a json file.
+
+Here is how a configuration file looks like-
+
+```ini
+src.type=json
+src.uri=/full/path/to/file.json
+
+dest.type=elasticsearch
+dest.uri=appname
+```
+
+The `file.json` should contain a json object with table namespaces as its keys.
+The keys in return should contain an array containing the data that belongs to that namespace.
+
+Example - 
+
+```js
+{
+	"pokemons": [
+		{
+			"_id": 1,
+			"name": "Raichu"
+		},
+		{
+			"_id": 1,
+			"name": "Bulbasaur"
+		}
+	],
+	"bitbeasts": [
+		{
+			"_id": 1,
+			"name": "Dragoon",
+			"owner": "Tyson"
+		},
+		{
+			"_id": 2,
+			"name": "Dranzer",
+			"owner": "Kai"
+		}
+	]
+}
+```