[BUG] README doesn't mention how to install the gem from Rubygems.org

[etagwerker]

Before we start...:

• [x] I checked the documentation and found no answer
• [x] I checked to make sure that this issue has not already been filed
• [x] I'm reporting the issue to the correct repository (for multi-repository projects)

Version, Branch, or Commit:

main branch.

Expected behavior:

README should mention how to install this gem.

Actual behavior:

README tells people to install from GitHub (which can be a bit risky)

Steps to reproduce:

Read our README.

Context:

dotenv_validator v1.0.0 has been released over here: https://rubygems.org/gems/dotenv_validator

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