Merge #1563: doc: Add convention for defaults

1668564 doc: Add convention for defaults (Tim Ruffing)

Pull request description:

ACKs for top commit:
  hebasto:
    ACK 1668564
  jonasnick:
    ACK 1668564

Tree-SHA512: cb359e20f1944b4095512413df4d55ccd89e1838da8e006cea4e523e6d1d5008afbab4e0d87b05a6e6ad991d1a0003066fc205d15443c8f1eb2b02e13196155a

CONTRIBUTING.md

@@ -49,6 +49,7 @@ In addition, libsecp256k1 tries to maintain the following coding conventions:
 * Operations involving secret data should be tested for being constant time with respect to the secrets (see [src/ctime_tests.c](src/ctime_tests.c)).
 * Local variables containing secret data should be cleared explicitly to try to delete secrets from memory.
 * Use `secp256k1_memcmp_var` instead of `memcmp` (see [#823](https://github.com/bitcoin-core/secp256k1/issues/823)).
+* As a rule of thumb, the default values for configuration options should target standard desktop machines and align with Bitcoin Core's defaults, and the tests should mostly exercise the default configuration (see [#1549](https://github.com/bitcoin-core/secp256k1/issues/1549#issuecomment-2200559257)).
 
 #### Style conventions