Add example of getInitialProps to with-reasonml (#5975)

The current reasonml needs an example of how to do getInitialProps.  tmepple posted the only known way of doing it in [this comment](https://github.com/zeit/next.js/issues/4202#issuecomment-439175214).   It is unlikely reasonml users are going to discover that comment or figure it out on their own so having it in an example is critical. 

Also, dependencies updated.  After updating dependencies, I get this error:
```
ModuleBuildError: Module build failed (from ./node_modules/next/dist/build/webpack/loaders/next-babel-loader.js):
[1] Error: Cannot find module '@babel/core'
[1]  babel-loader@8 requires Babel 7.x (the package '@babel/core'). If you'd like to use Babel 6.x ('babel-core'), you should install 'babel-loader@7'.
```
Seems like @babel/core is required as peer dependency per [this comment](https://github.com/babel/gulp-babel/issues/124#issuecomment-326853222) so I added it.

The way to do getInitialProps has changed in 7.0.2 canary so this also has a comment in the code about what change is required to get that working.

examples/with-reasonml/.bsb.lock

@@ -0,0 +1 @@
+16836

examples/with-reasonml/package.json

@@ -10,12 +10,13 @@
   "dependencies": {
     "babel-plugin-bucklescript": "^0.5.3",
     "next": "latest",
-    "react": "^16.4.2",
-    "react-dom": "^16.4.2",
+    "react": "^16.7.0",
+    "react-dom": "^16.7.0",
     "reason-react": "^0.5.3"
   },
   "devDependencies": {
-    "bs-platform": "^4.0.5",
-    "concurrently": "^4.0.1"
+    "@babel/core": "^7.2.2",
+    "bs-platform": "^4.0.14",
+    "concurrently": "^4.1.0"
   }
 }

examples/with-reasonml/pages/index.re

@@ -1,13 +1,44 @@
 let component = ReasonReact.statelessComponent("Index");
 
-let make = (_children) => {
+let make = (~onServer, _children) => {
   ...component,
-  render: (_self) =>
+  render: _self =>
     <div>
       <Header />
-      <p> (ReasonReact.string("HOME PAGE is here!")) </p>
+      <p>
+        {ReasonReact.string("HOME PAGE is here!")}
+        <br />
+        {ReasonReact.string("onServer: " ++ string_of_bool(onServer))}
+      </p>
       <Counter />
-    </div>
+    </div>,
 };
 
-let default = ReasonReact.wrapReasonForJs(~component, (_jsProps) => make([||]));
+let default = ReasonReact.wrapReasonForJs(~component, jsProps => make(~onServer=jsProps##onServer, [||]));
+
+/* The way to do getInitialProps: https://github.com/zeit/next.js/issues/4202#issuecomment-439175214 */
+let getInitialProps = context =>
+  Js.Promise.make((~resolve, ~reject as _) => {
+    let onServer =
+      switch (Js.Nullable.toOption(context##req)) {
+      | None => false
+      | Some(_) => true
+      };
+    resolve(. {"onServer": onServer});
+  });
+
+/* In 7.0.2 canary, you must remove the Js.Promise.make wrapper */
+/*
+ let getInitialProps = context => {
+   let onServer =
+     switch (Js.Nullable.toOption(context##req)) {
+     | None => false
+     | Some(_) => true
+     };
+   {"onServer": onServer};
+ };
+ */
+
+let inject = [%bs.raw {| (cls, fn) => cls.getInitialProps = fn |}];
+
+inject(default, getInitialProps);