[Doc] Add mail subscribe way in "Community" page & README

[imbajin]

WANTED FOR EVERYONE to take this task 📫

Also add https://lists.apache.org/list.html?dev@hugegraph.apache.org for users to read all the mail easily

• https://hugegraph.apache.org/community/ (community page, and the Chinese page need translate)
• https://hugegraph.apache.org/docs/contribution-guidelines/subscribe/ (how to subscribe mailing list)
• https://github.com/apache/incubator-hugegraph/issues/1749 (cn issue)

Update README to add these info:

• how to subscribe dev mail, and try to send it after 24h later if u don't receive the reply mail (in english)
• mailing list url for all to visit directly
• don't send mail to dev until u subscribe the mailing list successfully (otherwise the mail will be banned)

U could comment in this issue to address the improvement & submit a PR to link with it

[msgui]

Hi, @imbajin, I want to try. Please assign to me

[imbajin]

Hi, @imbajin, I want to try. Please assign to me

fine, u could enhance the website(community page) & translate chinese page

another pr #258 enhanced the README in doc repo, u could refer the description