Document default values for query variables graphql#294 (graphql#296)

* added a default variable paragraph to the documentation

* Updated the Default Variables section of the documentation

* Update Learn-Queries.md

Author: SheW0lf

site/learn/Learn-Queries.md

@@ -165,6 +165,23 @@ Variable definitions can be optional or required. In the case above, since there
 To learn more about the syntax for these variable definitions, it's useful to learn the GraphQL schema language. The schema language is explained in detail on the Schema page.
 
 
+### Default variables
+
+Default values can also be assigned to the variables in the query by adding the default value after the type declaration. 
+
+```graphql
+query HeroNameAndFriends($episode: Episode = "JEDI") {
+  hero(episode: $episode) {
+    name
+    friends {
+      name
+    }
+  }
+}
+```
+
+When default values are provided for all variables, you can call the query without passing any variables. If any variables are passed as part of the variables dictionary, they will override the defaults. 
+
 ## Operation name
 
 One thing we also saw in the example above is that our query has acquired an _operation name_. Up until now, we have been using a shorthand syntax where we omit both the `query` keyword and the query name, but in production apps it's useful to use these to make our code less ambiguous.