Recommend Let's Encrypt for SSL

[berezovskyi]

http://oslc.github.io/developing-oslc-applications/iotp_adaptor/ssl-support.html says that SSL certs are mostly paid and suggests to self-sign certs to get them for free.

Since 2016, Let's Encrypt has become the main provider of the free SSL certs around the world.. It is supported by Cisco, Akamai, Mozilla. The rule of a thumb nowadays is that unless you need EV or other strange things, you should use Let's Encrypt.

So, the advice should be:

Use Let's Encrypt to obtain free certificates unless you need special features such a EV.

[berezovskyi]

cc @jamsden