Add Luxon supported options documentation to README

[stockiNail]

This PR is adding documentation about which options can be set to configure Luxon.

As reported by PR #42 and issue #46, these could not be well-known.

[coveralls]

Pull Request Test Coverage Report for Build 2033284960

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 72.727%

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Totals
Change from base Build 1660678495:	0.0%
Covered Lines:	24
Relevant Lines:	31

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 2033284960

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 72.727%

---

Totals
Change from base Build 1660678495:	0.0%
Covered Lines:	24
Relevant Lines:	31

---

💛 - Coveralls

[stockiNail]

@benmccann thank you very much for your review. My English is not so good! I have changed INTL to Intl also in the other options description where it was used.

[benmccann]

thank you! looks like a nice change. I can't merge, so will need to wait for someone else for that