Outdated or missing READMEs for c entity examples.

[hokeun]

• The example section of this README does not include the instructions for building entities. We need a redirection to the correct README and additional explanation on how example entities are compiled and used.
• We need a proper README for this directory explaining what examples we have at a higher level and links to READMEs.
• This directory needs to be renamed to something like SST_testbed. Also, this might need a separate directory rather than being within the example directory.

[Jakio815]

Sure, I'll work on it.