Add section on reading files

docs/basic-features/data-fetching.md

@@ -94,6 +94,65 @@ export const getStaticProps: GetStaticProps = async context => {
 }
 ```
 
+### Reading files: Use `process.cwd()`
+
+Files can be read directly from the filesystem in `getStaticProps`.
+
+In order to do so you have to get the full path to a file.
+
+Since Next.js compiles your code into a separate directory you can't use `__dirname` as the path it will return will be different from the pages directory.
+
+Instead you can use `process.cwd()` which gives you the directory where Next.js is being executed.
+
+```jsx
+import fs from 'fs'
+import path from 'path'
+
+// posts will be populated at build time by getStaticProps()
+function Blog({ posts }) {
+  return (
+    <ul>
+      {posts.map(post => (
+        <li>
+          <h3>{post.filename}</h3>
+          <p>{post.content}</p>
+        </li>
+      ))}
+    </ul>
+  )
+}
+
+// This function gets called at build time on server-side.
+// It won't be called on client-side, so you can even do
+// direct database queries. See the "Technical details" section.
+export async function getStaticProps() {
+  const postsDirectory = path.join(process.cwd(), 'posts')
+  const filenames = fs.readdirSync(postsDirectory)
+
+  const posts = filenames.map(filename => {
+    const filePath = path.join(postsDirectory, filename)
+    const fileContents = fs.readFileSync(fullPath, 'utf8')
+
+    // Generally you would parse/transform the contents
+    // For example you can transform markdown to HTML here
+
+    return {
+      filename,
+      content: fileContents,
+    }
+  })
+  // By returning { props: posts }, the Blog component
+  // will receive `posts` as a prop at build time
+  return {
+    props: {
+      posts,
+    },
+  }
+}
+
+export default Blog
+```
+
 ### Technical details
 
 #### Only runs at build time