app: Create basic app.

app/src/components/header.js

@@ -0,0 +1,29 @@
+import { Link } from "gatsby"
+import PropTypes from "prop-types"
+import React from "react"
+
+const Header = ({ siteTitle }) => (
+  <header class="my-1 bg-red">
+      <h1 style={{ margin: 0 }}>
+        <Link
+          to="/"
+          style={{
+            color: `white`,
+            textDecoration: `none`,
+          }}
+        >
+          {siteTitle}
+        </Link>
+      </h1>
+  </header>
+)
+
+Header.propTypes = {
+  siteTitle: PropTypes.string,
+}
+
+Header.defaultProps = {
+  siteTitle: ``,
+}
+
+export default Header