Fastlane, main doc beta & release lanes

In the [main](https://docs.fastlane.tools/) docs page, the following 2 lanes are defined:

```ruby
lane :beta do
increment_build_number
build_app
upload_to_testflight
end

lane :release do
capture_screenshots
build_app
upload_to_app_store # Upload the screenshots and the binary to iTunes
slack # Let your team-mates know the new version is live
end
```

Is there a reason the release lane isn't including `increment_build_number`?

If a dev currently uses the following workflow:
1. Deploys beta
1. Tests
1. Deploys release

With the current script, they'll run into an issue where there is already an existing app with the same build # & version #. I suggest we add `increment_build_number` to the release lane, so as not to confuse devs into thinking that its handled automagically via release lanes somehow.