[WWW-25] Change documentation to include Let's Encrypt certificate usage

_MG on 2017-12-20T14:08:33Z says:_

Implementation of Let's Encrypt certificates for outbound server connection is possible. There are multiple reported instances. Please add instructions to documentations.
Compare for example [https://answers.tasktools.org/questions/5439720/answers/7241971] or [https://gist.github.com/polettix/e8007a7f2064e7f133d93e060032a880]

*Changes on taskd server config:*

server.cert=/path/to/letsencrypt-certificate-server.cert.pem

server.key=/path/to/letsencrypt-certificate-server.key.pem

*Changes on taskwarrior client config:*

taskd.ca=[Let's Encrypt X3 Intermediate certificate].pem (available at https://letsencrypt.org/certificates/)

Remaining configuration and setup stays the same. Depending on setup it could be good practise to link LE certificates in $TASKDATA (ln -sf) instead of directly referring them in config.