Improve installation instructions

[dunnkers]

1. 🔨 fixes the console script entry point (In other words: when you install fseval using pip or from the Git source, a fseval command should be available in your terminal. Now it is.).
2. 📄 adds documentation for a second installation method: installing from source. For those who do not want to use the PyPi package index, or just want the cutting-edge version of fseval.

• 🆕 New notice in the README:
  
• 🆕 New notice on the website:
  

🙌🏻

[codecov-commenter]

Codecov Report

Merging #81 (8c1b61c) into master (18453ea) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #81   +/-   ##
=======================================
  Coverage   98.15%   98.15%           
=======================================
  Files          81       81           
  Lines        2380     2380           
=======================================
  Hits         2336     2336           
  Misses         44       44           

Impacted Files	Coverage Δ
setup.py	100.00% <ø> (ø)

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