Update README.md

[keurek]

SUMMARY OF CHANGES

• update README.md to reflect ReEDS v2020

  • add weblink for open solver deck
  • update the ReEDS documentation reference from v2019 to v2020
  • update Table 1 to compare v2019 and v2020

• update README.md with general corrections

  • correct grammatical errors
  • revise FAQ responses for GAMS-related licensing questions