Add ellswift usage example #1551

   6 | @@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   7 |  
   8 |  ## [Unreleased]
   9 |  
  10 | +#### Added
  11 | + - Added usage example for a ElligatorSwift key exchange.

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  76 | +    /*** Create the shared secret on each side ***/
  77 | +
  78 | +    /* Perform x-only ECDH with seckey1 and ellswift_pubkey2. Should never fail
  79 | +     * with a verified seckey and valid pubkey. Note that both parties pass both
  80 | +     * EllSwift pubkeys in the same order; the pubkey to be actually used for the
  81 | +     * shared secret derivation is determined by the "party" boolean (sixth parameter). */

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 107 | +    /* This will clear everything from the context and free the memory */
 108 | +    secp256k1_context_destroy(ctx);
 109 | +
 110 | +    /* It's best practice to try to clear secrets from memory after using them.
 111 | +     * This is done because some bugs can allow an attacker to leak memory, for
 112 | +     * example through "out of bounds" array access (see Heartbleed), Or the OS

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   9 | @@ -10,6 +10,7 @@ ctime_tests
  10 |  ecdh_example
  11 |  ecdsa_example
  12 |  schnorr_example
  13 | +ellswift_example

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