Missing configuration for target attribute in documentation

[lribi]

In the readme a[target] is whitelisted but HTML purifier needs additional configuration to actually allow the target attribute.

https://github.com/Exercise/HTMLPurifierBundle#whitelist-attributes
http://htmlpurifier.org/docs
http://htmlpurifier.org/live/configdoc/plain.html#Attr.AllowedFrameTargets

This could lead to some wrong configurations.

Maybe adding Attr.AllowedFrameTargets to the example could improve this?

[HeahDude]

Hello @lribi, thank you for opening this issue and sorry for the late answer.
I agree we could improve the example of the README, would you like to submit a PR?

[lribi]

Hi @HeahDude

Sure, I'll make a PR when I find the time. Shouldn't take long.