Bugfix: Make sure the whole mail is read

[huangkevin404]

If a mail is bigger than 8192 bytes (e.g. there are a lot of headers), the io.LimitReader will truncate the mail and we will lose part of the mail body. That will cause decomposePayload to fail.

This PR replaces io.LimitReader(f, 8192) with os.ReadFile(path). os.ReadFile has been introduced in go 1.16, we bump the dependencies.

This PR also fixes the YAML link in the README.

[y-martin]

Hello @cherti

Can you check this ?

Regards