add documentation about BENTOML_HOME

[ALee008]

Place description about the need of env variable BENTOML_HOME more prominently in documentation. I had to parse the bentoml code to finally realize you had to export this env variable to change the BentoML repository path. Would be gread to add it the tutorial or the "Main Concepts" part of the documentation.

[tomcatmew]

very important info ! you save my life