Add readme for componentDidCatch example

examples/with-componentdidcatch/package.json

 {
-  "name": "hello-world",
+  "name": "with-componentdidcatch",
   "version": "1.0.0",
   "scripts": {
     "dev": "next",

examples/with-componentdidcatch/readme.md

+[![Deploy to now](https://deploy.now.sh/static/button.svg)](https://deploy.now.sh/?repo=https://github.com/zeit/next.js/tree/master/examples/with-componentdidcatch)
+
+# With `componentDidCatch` example
+
+## How to use
+
+### Using `create-next-app`
+
+Execute [`create-next-app`](https://github.com/segmentio/create-next-app) with [Yarn](https://yarnpkg.com/lang/en/docs/cli/create/) or [npx](https://github.com/zkat/npx#readme) to bootstrap the example:
+
+```bash
+npx create-next-app --example with-componentdidcatch with-componentdidcatch-app
+# or
+yarn create next-app --example with-componentdidcatch with-componentdidcatch-app
+```
+
+### Download manually
+
+Download the example [or clone the repo](https://github.com/zeit/next.js):
+
+```bash
+curl https://codeload.github.com/zeit/next.js/tar.gz/canary | tar -xz --strip=2 next.js-canary/examples/with-componentdidcatch
+cd with-componentdidcatch
+```
+
+Install it and run:
+
+```bash
+npm install
+npm run dev
+# or
+yarn
+yarn dev
+```
+
+Deploy it to the cloud with [now](https://zeit.co/now) ([download](https://zeit.co/download))
+
+```bash
+now
+```
+
+## The idea behind the example
+
+This example shows how to catch errors caught by `componentDidCatch` in React's client side.