CONTRIBUTING: add some coding and style conventions

CONTRIBUTING.md

@@ -39,6 +39,22 @@ Chat history logs can be found at https://gnusha.org/secp256k1/.
 
 The Contributor Workflow & Peer Review in libsecp256k1 are similar to Bitcoin Core's workflow and review processes described in Core's [CONTRIBUTING.md](https://github.com/bitcoin/bitcoin/blob/master/CONTRIBUTING.md).
 
+### Coding Conventions
+
+In addition, libsecp256 tries to maintain the following coding conventions:
+
+- No runtime heap allocation (e.g., no `malloc`).
+- The tests should cover all lines and branches of the library (see [Test coverage](#test-coverage)).
+- 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)).
+
+#### Style Conventions
+
+- Commits should be atomic and diffs should be easy to read. For this reason, do not mix any formatting fixes or code moves with actual code changes. Make sure each individual commit is hygienic: that it builds successfully on its own without warnings, errors, regressions, or test failures.
+- Use `void *ptr` instead of `void* ptr`.
+- Avoid trailing whitespaces.
+
 ### Test coverage
 
 This library aims to have full coverage of the reachable lines and branches.