Updating the README.md file

[JuanCab]

This PR consists of edits to the README.md file which are meant to:

• Provide better documentation as to the purpose of stellarphot
• Provide slightly expanded installation instructions (Addressing #333) to clarify how to handle the needed packages for exoplanet transit light curves.

I have not added instructions for installing stellarphot for developers for now.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 75.25%. Comparing base (96d98fb) to head (312fc0c). Report is 320 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #356 +/- ## ======================================= Coverage 75.25% 75.25% ======================================= Files 27 27 Lines 3600 3601 +1 ======================================= + Hits 2709 2710 +1 Misses 891 891 ```

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