Add README

README.md

 # Mime Normalizer
 
+Library implements normalisation of HTML and plain parts of a MIME message.
+
+Normalisation strategies documentation:
+https://code.vereign.com/light/documentation/-/blob/master/Validation.md#normalization-strategies
+
+## Testing
+
+HTML and text files with test cases provided in `__tests__/files`
+If you are going to create a new test case, please follow the structure inside this directory.
+
+- `yarn test`
+- `yarn test:watch`
+
+_Important note: MIME normaliser does not cover all provided test cases.
+Some of them explicitly ignored to indicate which tests has to be fixed. Please refer to `failingCases` arrays inside the test files._