Add ignoreTranslateChildren to docs.

[thchia]

Thank you for this library.

I noticed that this part is missing. I'm not sure if it is intentional or not, but here is a simple PR - feel free to close if it is intentional :)

In case you are wondering how I came across this 'hidden' feature:

I have requirements for both:

• an explicitly-defined default language and translations (fetched from external API resource)
• placeholder children for Translate for use in a test environment.

Therefore I wanted placeholders that I could query during tests without loading the actual full set of translations, but during normal usage I did not want these placeholders added to the fetched translations set.

[ryandrewjohnson]

@thchia good catch! That option was forgotten in the docs. I did manage to remember to add them to the upcoming v3 docs, but will merge this in for v2 docs in the meantime.