Update README

[pierrelalanne]

The goal of this PR is not to deep dive in to revamping the README, but simply to rename the env variable's name for GitGuardian API key to make it similar to the one used in ggshield.

[gitguardian]

️✅ There are no secrets present in this pull request anymore.

If these secrets were true positive and are still valid, we highly recommend you to revoke them.
Once a secret has been leaked into a git repository, you should consider it compromised, even if it was deleted immediately.
Find here more information about risks.

---

<sup><sub>🦉 GitGuardian detects secrets in your source code to help developers and security teams secure the modern development process. You are seeing this because you or someone else with access to this repository has authorized GitGuardian to scan your pull request.

Our GitHub checks need improvements? Share your feedbacks!</sub></sup>

[codecov-commenter]

Codecov Report

Merging #77 (28fe4c6) into master (3892245) will not change coverage.
Report is 3 commits behind head on master.
The diff coverage is 100.00%.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

@@           Coverage Diff           @@
##           master      #77   +/-   ##
=======================================
  Coverage   92.18%   92.18%           
=======================================
  Files           6        6           
  Lines         614      614           
=======================================
  Hits          566      566           
  Misses         48       48           

Flag	Coverage Δ
unittests	92.18% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
pygitguardian/__init__.py	100.00% <100.00%> (ø)
pygitguardian/client.py	79.82% <ø> (ø)