update documentation for v2

[mnishiguchi]

Description

This is a proposal for organizing migration guides. It mainly addresses two things.

1. Outdated contents in README.md
2. Introducing a new directory that holds our guides including migration instructions

Outdated contents in README.md

• README.md still talks as if v2 had not been released yet. So it needs updating.

Organization of our migration guides

• Currently our upgrade guides are located in two files:
  1. README.md
  2. PORTING.md
• Some projects out there seem to have a dedicated directory for "guides", which we could try now that we have more than one migration concerns

Notes

• The contents are mostly unchanged; basically they are just moved around
• Once we decide to go ahead, we will have to check the hyperlinks to PORTING.md since it is referenced in a few places; also included files in the package might need adjustment