Create README.md

Author: simonw

README.md

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+# puppeteer-screenshot
+
+Vercel app for taking screenshots of web pages using Puppeteer
+
+Original code by [@styfle](https://github.com/styfle) in [this pull request](https://github.com/vercel/now-examples/pull/207).
+
+I upgraded the code to work with the latest Vercel and added a `?key=` mechanism to protect it from unauthorized access.
+
+## Usage
+
+Set a Vercel secret called `SECRET_KEY` with a random string in it (I generated mine by running `uuidgen | md5`), then deploy.
+
+You can access screenshots for pages using:
+
+    https:/.../zeit.co/blog?type=png&key=...
+
+Screenshots can take several seconds to generate so it's a good idea to cache them somewhere.