Document variable translation from taxdata to cps_us data

**Story**:
As a developer, to make the raw cps us variables ready to use for modeling, I need to
1. understand how the variables are translated from survey to model
2. a way to test if my translation is correct

**Acceptance Criteria**:

1. Document the following information in `openfisca-data` repo in markdown
(1) Variables name mapping (raw name, family/household/person, mapped output name)
(2) Logic for each variable translation (just renaming or steps to process)
(3) Prioritization groups:
a. [important+easy]: Variables that are ready to translate with clear processing steps
b. [important+hard]: Variables that are important in the future but needs more efforts to clarify the processing steps
c. [not important]: Variables that are not important at the moment because it is not used in taxdata.
(4) Any reference and unanswered questions
2. Create Google Doc to organize links of all docs and share it with us developer group