* Add instructions for installing a TLS server certificate on nginx
* Fix link to nginx in README.md
* List all three ways to get a webserver TLS certificate. With some
hints on which to chose for which purpose.
* Do not add CSR instructions, because they can change over time and each CA may
have slightly different requirements.
* Add a hint about setting protocol selection.
* Fix typo in provider-setup.md
* Add technical hint about ERR_SSL_KEY_USAGE_INCOMPATIBLE.
* Improve format and spelling.
* Remove OSCP hint as this is related to client certificates.
* List all three ways to get a webserver TLS certificate. With some
hints on which to chose for which purpose.
* Refer to the official nginx documentation and remove some of the
instructions, because they can change over time and each CA may
have slightly different requirements.
* Add a hint about setting protocol selection.
