docs: general project description

Author: tomato42

docs/source/index.rst

@@ -6,6 +6,20 @@
 Welcome to python-ecdsa's documentation!
 ========================================
 
+``ecdsa`` implements
+`elliptic-curve cryptography (ECC) <https://en.wikipedia.org/wiki/Elliptic-curve_cryptography>`_,
+more specifically the
+`Elliptic Curve Digital Signature Algorithm (ECDSA) <https://en.wikipedia.org/wiki/Elliptic_Curve_Digital_Signature_Algorithm>`_
+and the
+`Elliptic Curve Diffie-Hellman (ECDH) <https://en.wikipedia.org/wiki/Elliptic-curve_Diffie%E2%80%93Hellman>`_
+algorithms.
+Both of those algorithms are used in many protocols in practice, like
+in
+`TLS <https://en.wikipedia.org/wiki/Transport_Layer_Security>`_
+or
+`SSH <https://en.wikipedia.org/wiki/Secure_Shell_Protocol>`_.
+
+
 .. toctree::
    :maxdepth: 2
    :caption: Contents: