Add API schema introspection docs

I.e. how to query for a GraphQL schema from its API.

Author: penyuan

contrib/schema introspection.md

@@ -0,0 +1,110 @@
+# Introspecting the Wikifactory GraphQL API schema.
+
+One could send a GraphQL query to an API to obtain its complete schema: 
+https://stackoverflow.com/a/46543255/186904
+
+Here's the query: 
+
+```
+query IntrospectionQuery {
+	__schema {
+		queryType {
+			name
+		}
+		mutationType {
+			name
+		}
+		subscriptionType {
+			name
+		}
+		types {
+			...FullType
+		}
+		directives {
+			name
+			description
+			locations
+			args {
+				...InputValue
+			}
+		}
+	}
+}
+
+fragment FullType on __Type {
+	kind
+	name
+	description
+	fields(includeDeprecated: true) {
+		name
+		description
+		args {
+			...InputValue
+		}
+		type {
+			...TypeRef
+		}
+		isDeprecated
+		deprecationReason
+	}
+	inputFields {
+		...InputValue
+	}
+	interfaces {
+		...TypeRef
+	}
+	enumValues(includeDeprecated: true) {
+		name
+		description
+		isDeprecated
+		deprecationReason
+	}
+	possibleTypes {
+		...TypeRef
+	}
+}
+
+fragment InputValue on __InputValue {
+	name
+	description
+	type {
+		...TypeRef
+	}
+	defaultValue
+}
+
+fragment TypeRef on __Type {
+	kind
+	name
+	ofType {
+		kind
+		name
+		ofType {
+			kind
+			name
+			ofType {
+				kind
+				name
+				ofType {
+					kind
+					name
+					ofType {
+						kind
+						name
+						ofType {
+							kind
+							name
+							ofType {
+								kind
+								name
+							}
+						}
+					}
+				}
+			}
+		}
+	}
+}
+```
+
+The results are stored in `WIF_schema.json`, and converted for use with the [`sgqlc`](https://github.com/profusion/sgqlc) library into `WIF_schema.py`.