doc: add coding guidelines

Signed-off-by: Stephan Mueller <smueller@chronox.de>

Author: smuellerDD

coding-guidelines.md

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+# Code Structure
+
+Due to the fact that leancrypto is compiled for many different execution environments, the compilation process must consider the specifics of each target execution environment. The leancrypto code implements target-specific code in independent source code files. Depending on the target environment, the source code files are or are not compiled.
+
+## Modules
+
+The leancrypto code contains different modules which are encapsulated into different directories.
+
+The following modules are present:
+
+* `aead`: Implements AEAD symmetric algorithms.
+
+* `apps`: Applications provided with leancrypto
+
+* `asn1`: ASN.1, X.509 and PKCS#7/CMS code
+
+* `bike`: Code-based PQC algorithm of BIKE
+
+* `curve25519`: Curve25519 implementation including EdDSA and ECDH
+
+* `doc`: Doxygen generator
+
+* `drng`: Deterministic random number generator including the seeded_rng implemenation with its link code to entropy sources.
+
+* `efi`: Compilation support for EFI environment
+
+* `hash`: hash implemenations of SHA2, SHA3, Ascon
+
+* `hmac`: HMAC implementation
+
+* `internal`: Support code
+
+* `kdf`: Different key derivation functions
+
+* `kmac`: SP800-185 KMAC
+
+* `linux_kernel`: Linux kernel compilation support and code to register leancrypto with the kernel crypto API
+
+* `ml-dsa`: FIPS 204 ML-DSA PQC algorithm
+
+* `ml-kem`: FIPS 203 ML-KEM PQC algorithm
+
+* `otp`: One-Time-Pad algorithms
+
+* `slh-dsa`: FIPS 205 SLH-DSA algorithm
+
+* `sym`: symmetric encryption algorithms with their block chaining modes
+
+## Module Directories
+
+The individual module directories have a common structure:
+
+* `api`: Header files used in other parts of leancrypto or defining official APIs
+
+* `doc`: Algorithm specifications
+
+* `src`: Actual implementation
+
+* `tests`: Unit tests for the implementation
+
+## Coding Guidelines
+
+Code changes should follow these guidelines:
+
+* KNF code format (use `addon/clang-format-helper.sh`)
+
+* All externally visible symbols are prefixed with `lc_`.
+
+* All externally visible macros are prefixed with `LC_`.
+
+* All header files defining external APIs must have a name starting with `lc_` and cannot include internal header files.
+
+* Ifdef's in the C code should be reduced to an absolute minimum. Conditional compilation is defined by the build environment. If a C file that can be conditionally compiled provides services to other C files, the associated header file shall contain ifdef'ed NOOP functions.