Update README.md to include License and Slack (#841)

We need to make the software and the community more accessible.

README.md

@@ -1,7 +1,9 @@
 # NativeLink
 
+[![License](https://img.shields.io/badge/License-Apache_2.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)
 [![OpenSSF Scorecard](https://api.securityscorecards.dev/projects/github.com/TraceMachina/nativelink/badge)](https://securityscorecards.dev/viewer/?uri=github.com/TraceMachina/nativelink)
 [![OpenSSF Best Practices](https://www.bestpractices.dev/projects/8050/badge)](https://www.bestpractices.dev/projects/8050)
+[![Slack](https://img.shields.io/badge/slack--channel-blue?logo=slack)](https://nativelink.slack.com/join/shared_invite/zt-281qk1ho0-krT7HfTUIYfQMdwflRuq7A#/shared-invite/email)
 
 NativeLink is an extremely (blazingly?) fast and efficient build cache and
 remote executor for systems that communicate using the [Remote execution