libuhttpmock: Expand instructions for generating TLS certificate
libuhttpmock: Expand instructions for generating TLS certificate

This covers how the existing certificate was generated. Given issue #5,
this approach might not still be correct.

Signed-off-by: Philip Withnall <29c2a2ac2a854b9cf047275aa9de346d096be2e0@endlessm.com>

pwithnall/uhttpmock,pwithnall/uhttpmock