Update README.rst with supported alternatives for simp_le

[meeuw]

This PR extends #119 with a link to scripts to migrate your account/configuration to certbot. Certbot is the official Let’s Encrypt client which can be used in a certonly mode which is quite compatible with simp_le.

https://github.com/meeuw/simp_le2certbot

[zenhack]

@meeuw, fwiw per #119 my fork is maintained and has been the one pointed to by letsencrypt.org for a couple years now. I don't expect this PR to receive a response; the original maintainer has been inactive on GitHub for almost 2 and a half years.

[meeuw]

@zenhack I know and I really appreciate what you've done for simp_le (I've used your fork for some time). But don't you think it would be better to advice users to use certbot? What your fork (and simp_le) are missing is:

• ACME v2 support
• Linux distribution packages (so people can just use their package manager to keep their letsencrypt client up to date)

It doesn't seems like you or @kuba has time to fix this.