fixed slack invitation link in README.md
fixed slack invitation link in README.md

Used a permanent invitation link in the README. This uglifies the
text version of the readme a bit, but the link never experies as
oppose to using tiny URLs.

Fixes: https://github.com/firecracker-microvm/firecracker/issues/1298

Signed-off-by: Andreea Florescu <f68a7d732e796861f8e79ea704b43a5ee52eddaf@amazon.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker