Added documentation for how to add a custom query to filter nodes by a connected node's properties (aws#130)

Added documentation for how to add a custom query to filter nodes by a connected node's properties.

Author: andreachild

CHANGELOG.md

@@ -70,6 +70,8 @@ permissions and limitations under the License.
   neptune analytics graph and added testing
   documentation ([#113](https://github.com/aws/amazon-neptune-for-graphql/pull/113))
 * Updated query and mutation names to be less graph-specific ([#124](https://github.com/aws/amazon-neptune-for-graphql/pull/124))
+* Documented custom query to filter nodes by connected nodes'
+  properties ([#130](https://github.com/aws/amazon-neptune-for-graphql/pull/130))
 
 ### Bug Fixes
 

README.md

@@ -289,6 +289,33 @@ type Mutation {
 }
 ```
 
+You can add a custom query to filter nodes based on a connected node's
+properties. This is an example of a query which can retrieve airports that have
+routes to a given country.
+
+```graphql
+type Query {
+    getAirportsWithRoutesToCountry(country: String): [Airport] @graphQuery(statement: "MATCH (getAirportsWithRoutesToCountry_Airport:`airport`)-[:route]->(toAirport:`airport` {country: '$country'}) WITH DISTINCT getAirportsWithRoutesToCountry_Airport")
+}
+```
+
+With this customized query, you can use a graphQL query to retrieve airports
+that have routes to CA and the CA airport properties:
+
+```graphql
+query GetAirportsWithRoutesToCountry {
+    getAirportsWithRoutesToCountry(country: "CA") {
+        city
+        code
+        country
+        airportRoutesOut(filter: {country: {eq: "CA"}}) {
+            city
+            code
+        }
+    }
+}
+```
+
 You can add a query or mutation using a Gremlin query. At this time Gremlin
 queries are limited to return *scalar* values, or *elementMap()* for a single
 node, or *elementMap().fold()* for a list of nodes.
@@ -389,6 +416,12 @@ For Example:
     "action": "add",
     "value": "outboundRoutesCountAdd: Int @graphQuery(statement: \"MATCH (this)-[r:route]->(a) RETURN count(r)\")"
   },
+  {
+    "type": "Query",
+    "field": "getAirportsWithRoutesToCountry",
+    "action": "add",
+    "value": "getAirportsWithRoutesToCountry(country:String): [Airport] @graphQuery(statement: \"MATCH (getAirportsWithRoutesToCountry_Airport:`airport`)-[:route]->(toAirport:`airport` {country: '$country'}) WITH DISTINCT getAirportsWithRoutesToCountry_Airport\")"
+  },
   {
     "type": "Mutation",
     "field": "deleteVersion",