feat(configuration): Add docs for select field on HTTP and gRPC direc…

…tives (#527)

docs/directives.md

@@ -839,6 +839,37 @@ type Query {
 }
 ```
 
+### select
+
+You can use `select` with mustache syntax to re-construct the directives
+response to the desired format. This is useful when data are deeply
+nested or want to keep specific fields only from the response.
+
+- EXAMPLE 1: if we have a call that returns `{ "user": { "items": [...],
+... } ... }` we can use `"{{.user.items}}"`, to extract the `items`.
+- EXAMPLE 2: if we have a call that returns `{ "foo": "bar", "fizz": {
+"buzz": "eggs", ... }, ... }` we can use `{ foo: "{{.foo}}", buzz:
+"{{.fizz.buzz}}" }`
+
+```graphql showLineNumbers
+type Query {
+  userCompany(id: Int!): Company
+    @grpc(
+      method: "news.UsersService.GetUserDetails"
+      select: "{{.company}}"
+    )
+  userDetails(id: Int!): UserDetails
+    @grpc(
+      method: "news.UsersService.GetUserDetails"
+      select: {
+        id: "{{.id}}"
+        city: "{{.address.city}}"
+        phone: "{{.phone}}"
+      }
+    )
+}
+```
+
 ## @http Directive
 
 The `@http` directive indicates a field or node relies on a REST API. For example:
@@ -1034,6 +1065,37 @@ type Query {
 }
 ```
 
+### select
+
+You can use `select` with mustache syntax to re-construct the directives
+response to the desired format. This is useful when data are deeply
+nested or want to keep specific fields only from the response.
+
+- EXAMPLE 1: if we have a call that returns `{ "user": { "items": [...],
+... } ... }` we can use `"{{.user.items}}"`, to extract the `items`.
+- EXAMPLE 2: if we have a call that returns `{ "foo": "bar", "fizz": {
+"buzz": "eggs", ... }, ... }` we can use `{ foo: "{{.foo}}", buzz:
+"{{.fizz.buzz}}" }`
+
+```graphql showLineNumbers
+type Query {
+  userCompany(id: Int!): Company
+    @http(
+      path: "/users/{{.args.id}}"
+      select: "{{.company}}"
+    )
+  userDetails(id: Int!): UserDetails
+    @http(
+      path: "/users/{{.args.id}}"
+      select: {
+        id: "{{.id}}"
+        city: "{{.address.city}}"
+        phone: "{{.phone}}"
+      }
+    )
+}
+```
+
 ## @js Directive
 
 The `@js` directive allows you to use JavaScript functions to resolve fields in your GraphQL schema. This can be useful