docs: Create README

Author: jackylamhk

README.md

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+# python-http-csp
+HTTP Content Security Policy Manager
+
+A library to make parsing and generating CSP policies a little easier.
+
+Read more about Content Security Policies: [https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP](https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP)
+
+
+## Using this library
+
+Parsing a CSP string:
+
+```python
+policy = CSP("default-src 'self';")
+```
+returns a CSP object with a `default_src` attribute with value `["'self'"]`.
+
+Generating a CSP string:
+
+```python
+policy = CSP()
+policy.default_src = ["'self'"]
+policy.image_src = ["*"]
+policy.media_src = ["example.org", "example.net"]
+generated_policy = policy.generate()
+```
+returns a string with the value `default-src 'self'; img-src *; media-src example.org example.net;`.