Add Contributing Guidelines

This is not obvious for someone who wants to contribute how to setup properly dev environment and run tests.
New section in documentation would help.
Example for that:

• https://github.com/splunk/addonfactory-ucc-generator/blob/main/README.md?plain=1
• https://github.com/splunk/addonfactory-ucc-generator/blob/main/docs/contributing.md?plain=1

It seems that the documentation should contain information about following prereq steps to run integration tests:

• export SPLUNK_HOME=/opt/splunk/
• export SPLUNK_ETC=/opt/splunk/etc/ - this one may be optional
• sudo cp -r tests/integration/data/solnlib_demo $SPLUNK_HOME/etc/apps
• sudo cp -r solnlib $SPLUNK_HOME/etc/apps/solnlib_demo/bin/
• sudo /opt/splunk/bin/splunk set servername custom-servername -auth admin:[splunk_password]
• sudo /opt/splunk/bin/splunk restart

Start with the README and the contributing documentation examples linked in the issue, then inspect tests/integration and tests/integration/data/solnlib_demo. Document the prerequisites and listed environment, copy, authentication, and restart commands for running integration tests. Done means a newcomer can set up the environment and follow the documented test process.