add quickstart docs for Example (#163)

* add quickstart docs for Example

* Update README.md

* Update README.md

* Apply suggestions from code review

Example/README.md

@@ -0,0 +1,49 @@
+# HCaptcha Example App
+
+This app is designed to let you quickly experiment with HCaptcha SDK options.
+
+You can also simply copy the example code in the ViewController to get up and running quickly in your own app.
+
+
+## Quickstart: Building the Example
+
+If you don't already have a working CocoaPods environment, we recommend homebrew:
+
+`/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"`
+
+Then install CocoaPods:
+
+`brew install cocoapods`
+
+Finally, install the pods needed for the Example app:
+
+`pod install`
+
+
+You should now be able to build and run the example in Xcode.
+
+
+## Example Changes
+
+### Switching to a Visible Checkbox
+
+You can try switching the 'size' parameter or any other by editing Example/HCaptcha/ViewController.swift
+
+For example:
+
+```
+@@ -175,7 +176,7 @@ class ViewController: UIViewController {
+         }
+
+         // swiftlint:disable:next force_try
+-        hcaptcha = try! HCaptcha(locale: locale)
++        hcaptcha = try! HCaptcha(locale: locale, size: .normal)
+```
+
+Tapping "Validate" will now show a visible checkbox, which can be tapped to continue the interaction.
+
+### Changing the Example Sitekey
+
+This example app sets the sitekey in Example/HCaptcha/Info.plist via the `HCaptchaKey` field.
+
+You can replace it with your own sitekey to try a different sitekey configuration, e.g. Passive mode.