Use RichText object for adding rich text blocks (#8675)
Use RichText object for adding rich text blocks (#8675)

I have just tried to follow the original instructions and got an `AttributeError: 'str' object has no attribute 'source'` when trying to append a rich text block to a `StreamField` programmatically. 

Wrapping the value in a `RichText` object solved the issue. This seems to be in line with the behavior described further down the page.
wagtail/wagtail,thenewguy/wagtail,wagtail/wagtail,wagtail/wagtail,zerolab/wagtail,rsalmaso/wagtail,zerolab/wagtail,thenewguy/wagtail,wagtail/wagtail,rsalmaso/wagtail,thenewguy/wagtail,thenewguy/wagtail,zerolab/wagtail,rsalmaso/wagtail,rsalmaso/wagtail,rsalmaso/wagtail,wagtail/wagtail,thenewguy/wagtail,zerolab/wagtail,zerolab/wagtail