Executing a script from the master branch is potentially unsafe

[ximaera]

https://github.com/google/tsunami-security-scanner/blame/6314ba862884833be797e7e40d97c5b2cfc32101/README.md#L42

Executing scripts from the master branch on the target (production) system is generally considered a bad practice because:

1. The master/main branch is a routinely updated working branch which at times may not have the bugfixes developed for a release. Therefore, scripts in it might generally contain code which is not properly tested.

2. The quick_start.sh script does path manipulation. Bugs in path manipulation are known to happen from time to time, with disastrous effects: 1, 2, 3, 4.
   That might potentially be prevented by a sanitizing githook but there are no such hooks in the repository.

3. The master branch would probably get renamed in the future. Hard-coding a link to it into README.md would unnecessarily complicate the transition with backward compatibility issues.

A suggestion: tag a stable release and link to it instead.