Add mutation to add keywords (#24)

src/schema.graphql

@@ -404,4 +404,5 @@ type Mutation {
   ): Product
   productGroup(productId: Int!, groupIds: [Int]): Product
   productKeywords(productId: Int!, keywordIds: [Int]): Product
+  addKeyword(name: String!, type: KeywordType): Keyword
 }