Add README

Author: benjie

README.md

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+<p align="center">
+  <a href="https://www.graphile.org/postgraphile/">
+    <img alt="PostGraphile" src="https://www.graphile.org/images/postgraphile.optimized.svg" width="60" />
+  </a>
+  <img alt="Graphile Heart" src="https://www.graphile.org/images/graphile.optimized.svg" width="60" />
+  <a href="https://www.gatsbyjs.org">
+    <img alt="Gatsby" src="https://www.gatsbyjs.org/monogram.svg" width="60" />
+  </a>
+</p>
+
+<h1 align="center">
+  gatsby-source-pg
+</h1>
+
+This module helps you pull your PostgreSQL database into Gatsby with minimal effort.
+
+To install:
+
+```
+yarn add gatsby-source-pg
+```
+
+(or `npm install --save gatsby-source-pg`)
+
+Then add the config to your `gatsby-config.js`:
+
+```js
+module.exports = {
+  /* ... */
+  plugins: [
+    /* ... */
+
+    {
+      resolve: "gatsby-source-pg",
+      options: {
+        connectionString: "postgres:///mydb",
+        schema: "public",
+        refetchInterval: 60, // Refetch data every 60 seconds
+      },
+    },
+  ],
+};
+```
+
+The `connectionString` can be any valid PostgreSQL connection string, a full
+connection string might look like:
+`postgres://pg_user:pg_pass@pg_host:pg_port/pg_db?ssl=1`
+
+## How to query
+
+```graphql
+{
+  postgres {
+    allPostsList {
+      id
+      authorId
+      userByAuthorId {
+        id
+        username
+      }
+      title
+    }
+  }
+}
+```
+
+## Example
+
+For a working example of `gatsby-source-pg`, see
+[gatsby-source-pg-example](https://github.com/graphile/gatsby-source-pg-example).
+
+## Thanks
+
+This plugin uses
+[`gatsby-source-graphql`](https://github.com/gatsbyjs/gatsby/tree/master/packages/gatsby-source-graphql#readme)
+to merge the PostGraphile GraphQL schema into Gatsby's.
+
+## Customising
+
+This plugin is powered by PostGraphile, which is built on the highly flexible
+and customisable Graphile Engine.
+
+You can add to `options` most of the [PostGraphile schema-only options](https://www.graphile.org/postgraphile/usage-schema/#api-createpostgraphileschemapgconfig-schemaname-options)
+
+In addition, we accept the `typeName` and `fieldName` options from
+`gatsby-source-graphql` which affect how the schema is namespaced, and the `refetchInterval` setting to trigger refetching data every X seconds.
+
+A common thing you might want to do is to shorten the names that PostGraphile uses by default, you can do this using a plugin such as `@graphile-contrib/pg-simplify-inflector`:
+
+```js
+// gatsby-config.js
+module.exports = {
+  /* ... */
+  plugins: [
+    /* ... */
+    {
+      resolve: "gatsby-source-pg",
+      options: {
+        connectionString: "postgres:///mydb",
+        schema: "public",
+
+        /* 👇 */
+        appendPlugins: [require("@graphile-contrib/pg-simplify-inflector")],
+        /* 👆 */
+      },
+    },
+  ],
+};
+```
+
+## Getting help
+
+Pop into the Graphile Discord: [http://discord.gg/graphile](http://discord.gg/graphile)
+
+## Helpful links
+
+- [Gatsby documentation](https://www.gatsbyjs.org/)
+- [gatsby-source-pg-example](https://github.com/graphile/gatsby-source-pg-example/)
+- [PostGraphile documentation](https://www.graphile.org/postgraphile/)