Declare if extension is safe for parallel reading

[explode]

Doing a Sphinx build using multiple processes will complain:

WARNING: the sphinx_bootstrap_theme extension does not declare if it is safe for parallel reading, 
assuming it isn't - please ask the extension author to check and make it explicit
WARNING: doing serial read

setup(app) simply needs to return {'parallel_read_safe':True} but should it be set True or False?

Sphinx API docs