Readme: Update user profile page instructions

[glebm]

Updates the instructions to use the Rails 5 render-anywhere method.

@jayroh @timdiggins What do you think?

[jayroh]

LGTM, man! 👍

[timdiggins]

@glebm what's the benefit of using this method rather than previous way? If I understand correctly this feature was created for rendering outside controllers rails/rails#18546

(I'm successfully using the previously documented mechanism)

[glebm]

@timdiggins The previous mechanism required the user to include Thredded::ApplicationHelper into the controller that renders the user's profile page, which had a high likelihood of name clashes with main_app (since we use helpers such as user_link).

[glebm]

@timdiggins and yeah, this was created for rendering outside of the controllers and fakes the request environment. But it's OK I guess as we don't use the request environment when rendering these partials.