feat: add doc/mail.md

doc/mail.md

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+# Setup mail
+
+* update Apt repository cache
+```
+sudo apt update
+```
+
+* install
+```
+sudo apt install mailutils msmtp --no-install-recommends
+```
+
+* study
+```
+https://unix.stackexchange.com/questions/640296/configuring-mail-command-to-use-msmtp
+https://moritzvd.com/email-with-smtp-debian-ubuntu/
+https://caupo.ee/blog/2020/07/05/how-to-install-msmtp-to-debian-10-for-sending-emails-with-gmail/
+https://phoenixnap.com/kb/linux-mail-command
+```
+
+* foo
+NOTE: read [this](https://superuser.com/questions/351841/how-do-i-set-up-the-unix-mail-command)
+```
+sudo apt install mailutils msmtp --no-install-recommends
+emacs ~/.msmtprc
+emacs ~/.mailrc
+emacs ~/.netrc
+chmod 600 ~/.netrc 
+```
+
+* foo
+```
+sudo apt install mailutils --no-install-recommends
+sudo apt install mailutils msmtp --no-install-recommends
+emacs ~/.msmtprc
+sudo apt-get install msmtp msmtp-mta
+sudo vi /etc/msmtprc
+sudo apt-get install bsd-mailx
+sudo vi /etc/mail.rc
+ ```
+
+* test
+```
+echo "Message" | mail -s "Title" sbegerad@posteo.de
+echo "Message" | mail -s "Title" dialog@swingbe.de
+ ```