Improve README to point out specific implementation details used in this repo

[amm-efabless]

To help a user properly understand what it takes to make a caravel_user_project into an IP-ready project like this one, ideally this repo's README should itemize some of what was done, why it's necessary, and where it might be variable, including:

• Makefile changes to support IPM
• openlane/SRAM_1024x32 -- what it actually represents in this repo; what it is, what it isn't, why it's done this way.
• config.json -- What needs to be added, why those things matter, and how to adjust them (e.g. obstructions) to suit requirements.
• pdn.tcl -- Is it needed, and why?