Add how to save the imageAsset

[map7]

I've spent a lot of time trying to find this information as I wanted to save a thumbnail and thought the best place for it would be in the nativescript-camera documentation.

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• All existing tests are passing
• Tests for the changes are included

What is the current behavior?

What is the new behavior?

Fixes/Implements/Closes #[Issue Number].

[cla-bot]

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CLA has not been signed by users: @map7.
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[zbranzov]

Hi @map7,
As you don't have signed CLA and since the PR is addressed from fork, which prevents our CI to pass, I will open a new PR with your changes. Thanks for contributing. #216