Added a README with some references

Author: liujed

README.md

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+Rack middleware for logging HTTP request–response pairs to a HAR file.
+
+References used when building this:
+  * https://ieftimov.com/post/writing-rails-middleware/
+  * https://github.com/rack/rack/blob/master/SPEC.rdoc
+  * https://w3c.github.io/web-performance/specs/HAR/Overview.html
+  * http://www.softwareishard.com/blog/har-12-spec/