Adding readme files to examples

examples/echo/README.md

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+### Important notes
+
+This example should be able to work with noise-c echo example.
+Echo constants required for proper encoding of Noise Protocol name were ported directly from noise-c.
+
+More info on noise-c echo: https://rweather.github.io/noise-c/example_echo.html 
+
+### Usage
+TBD

examples/noise-keys/README.md

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+Keypairs in this directory were generated solely for usage in examples. Private keys are not protected with a passphrase. 
+
+**DO NOT** use them for any other purpose than testing, as they are publicly available and it will pose a security threat to you.