Update README with guides for GCC evaluation and docker-free compilations

[jeongyooneo]

• Add a user guide for GCC evaluation: How to build GCC, configure to run GCC and getting the packet- and frame-level QoE metrics
• Guide on build options: Recommend source build for quick local development, and docker-based build for deployment
• Describe the merits of automated, reproducible testing of RL-based CC and GCC using the end-to-end peerconnection_serverless app in AlphaRTC. (convenience of not needing the signaling server, automated grading of QoE metrics, etc)