Add README

Author: Pytal

Diff for: README.md

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+# graphql-codegen-join-documents
+A plugin for [GraphQL Code Generator](https://graphql-code-generator.com/docs/getting-started/index) to join GraphQL queries/mutations/subscriptions together into a single file.
+
+## Usage
+Install the plugin:
+``` bash
+npm add -D graphql-codegen-join-documents
+```
+
+Enter the following into `codegen.yaml`:
+``` yaml
+schema: schema.graphql
+documents: src/**/*.gql
+
+generates:
+  queries.gql:
+    plugins:
+      - graphql-codegen-join-documents
+```
+
+Run codegen:
+``` bash
+npx graphql-codegen
+```