Detailed install instructions in the README

I'm trying to use this with `mu4e`, it often bugs me that when I join a thread of comments, my email has to overwrite the html with converted plaintext.

I was quite clueless on how to install this, but eventually found out and wrote it up.

So I managed to compile the binary helper, but what now? How do I open emails with this in `mu4e`? Is there something similar to the `org-mu4e-compose-org-mode` function?

Note the **TODO:** add instructions on how to actually edit emails in `gnus` and `mu4e`.