Mention the external tools in the "about" page

[facundobatista]

Including:

• https://github.com/erickgnavar/linkode.el
• https://pypi.org/project/linkodeit/

[c0x6a]

I created a Telegram bot to use Linkode: https://gitlab.com/c0x6a/linkode-bot (bot is in alpha state)

[matiasdemarchi]

Contact info now is up-to-date, and External Tools section were added. We can close this issue.

Link for PR: #157