Updates to the readme to clarify best practices and how to configure the plugin

[AnujRNair]

Summary

Updates to the readme to clarify best practices and how to configure the plugin As suggested in https://github.com/slackhq/csp-html-webpack-plugin/issues/77

Requirements (place an x in each [ ])

• [x] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've written tests to cover the new code and functionality included in this PR.
• [x] I've read, agree to, and signed the Contributor License Agreement (CLA).

[codecov[bot]]

Codecov Report

Merging #78 (6f20e8b) into master (c618ffd) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #78   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            2         2           
  Lines          108       108           
  Branches        19        19           
=========================================
  Hits           108       108           

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[maudnals]

Looking great :)