DOC Document adding new linking option phone to TinyMCE

[Oliver996]

Issues

• NEW: Add phone link option to TinyMCE sslink silverstripe-htmleditor-tinymce#28

PRs

• NEW Added sslink phone plugin to TinyMCE silverstripe-htmleditor-tinymce#29
• NEW Add phone link form for TinyMCE link option phone silverstripe-admin#2029

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[GuySartorelli]

I'm on the fence about giving this its own section with a screenshot since this is something people are likely to miss and it's a nice content author feature - but we can always do that later. For now this is perfectly fine.

Thanks!