CONTRIBUTING: mention credentials (#9599)
CONTRIBUTING: mention credentials (#9599)

For localstack, mention the required credentials

/cc @BusyJay 

### What is changed and how it works?

Document localstack credentials

### Check List <!--REMOVE the items that are not applicable-->

Tests 
- No code

### Release note <!-- bugfixes or new feature need a release note -->

None
BusyJay/tikv,c4pt0r/tikv,BusyJay/tikv,BusyJay/tikv,BusyJay/tikv