arm: document arm asm rules

Limiting the valid syntax to the subset supported by llvm 6.0 allows the direct use of recent Android and ios system toolchains.

@mstorsjo was were anything important added in the internal arm 32-bit assembler in llvm 5.0 or 6.0? If not, we don't have to be that restrictive.

CONTRIBUTING.md

@@ -9,7 +9,8 @@ The codebase is developed with the following assumptions:
 
 For the library:
 - C language with C99 version, without the VLA or the Complex (*\_\_STDC_NO_COMPLEX__*) features, and without compiler extension,
-- asm in .asm files, using the NASM syntax,
+- x86 asm in .asm files, using the NASM syntax,
+- arm/arm64 in .S files, using the GAS syntax limited to subset llvm 5.0's internal assembler supports,
 - no C++ is allowed, whatever the version.
 
 For the tools and utils: