Add warning messages in Readme

[seokho-son]

Add warning messages.

• Be careful!

  • Contributors should not push files related to their personal credencials (e.g., credentials.conf) to remote repository.
  • The credential file for CSPs (src/testclient/scripts/credentials.conf) is in the .gitignore condition.
  • So, credentials.conf will not be staged for a commit.
  • Anyway, please be careful.

• Be aware!

  • Be aware that creating VMs on public CSPs such as AWS, GCP, Azure, etc. may be billed.
  • With the default setting of testSet.env, TestClouds (TestCloud01, TestCloud02, TestCloud03) will be used to create mock VMs.
  • TestCloud01, TestCloud02, TestCloud03 are not real CSPs. It is used for testing purpose. (not support SSH into VM)
  • Anyway, please be aware cloud usage cost when you use public CSPs.

• Be aware!

  • If you created MCIS (VMs) for testing in public clouds, the VMs may be charged.
  • You need to termiate MCIS by using clean-all to avoid unexpected billing.
  • Anyway, please be aware cloud usage cost when you use public CSPs.

[seokho-son]

I added some warning messages according to @hermitkim1's advice.