#460 Organize contribution manual for code contribution and project management

[Hiroshi Miura]

Our contribution manual has several different discussion items; code contribution guide, internal guide, ADR, and management procedures.
I propose to organize technical contribution part and management part to seperare section.
It is better to seperare ADR from a process decition records. because ADR is for architecture design.

PR#2371 is a proposal to add PDR#0001 to express a motivation of document organization.