Add Contributing Guidelines

[hsekowski-splunk]

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