Clean up and complete RTD documentation

readthedocs/extra/developing/philosophy.rst

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+==========
+Philosophy
+==========
+
+
+The intention of the library is to have an existing MTProto library
+existing with hardly any dependencies (indeed, wherever Python is
+available, you can run this library).
+
+Being written in Python means that performance will be nowhere close to
+other implementations written in, for instance, Java, C++, Rust, or
+pretty much any other compiled language. However, the library turns out
+to actually be pretty decent for common operations such as sending
+messages, receiving updates, or other scripting. Uploading files may be
+notably slower, but if you would like to contribute, pull requests are
+appreciated!
+
+If ``libssl`` is available on your system, the library will make use of
+it to speed up some critical parts such as encrypting and decrypting the
+messages. Files will notably be sent and downloaded faster.
+
+The main focus is to keep everything clean and simple, for everyone to
+understand how working with MTProto and Telegram works. Don't be afraid
+to read the source, the code won't bite you! It may prove useful when
+using the library on your own use cases.