CodingStyle: Add notes on language and testing.

CodingStyle

@@ -1,6 +1,74 @@
+
+In general, use the same coding style as the surrounding code.
+
+However, do not make any unnecessary changes as that complicates
+the VCS (git) history and makes it harder to merge patches. So
+do not modify code just to make it conform to a coding style.
+
     Indentation
 
 Turn on a "fill tabs with spaces" option in your editor.
 
-Be also careful that some files are indented with 2 spaces (when they
+Remove tabs and trailing spaces from any lines that are modified.
+
+Note that some files are indented with 2 spaces (when they
 have large indentation) while most are indented with 4 spaces.
+
+    Language
+
+TCC is mostly implemented in C90. Do not use any non-C90 features
+that are not already in use.
+
+Non-C90 features currently in use, as revealed by
+./configure --extra-cflags="-std=c90 -Wpedantic":
+
+- long long (including "LL" constants)
+- inline
+- very long string constants
+- assignment between function pointer and 'void *'
+- "//" comments
+- empty macro arguments (DEF_ASMTEST in i386-tok.h)
+- unnamed struct and union fields (in struct Sym), a C11 feature
+
+    Testing
+
+A simple "make test" is sufficient for some simple changes. However,
+before committing a change consider performing some of the following
+additional tests:
+
+- Build and run "make test" on several architectures.
+
+- Build with ./configure --enable-cross.
+
+- If the generation of relocations has been changed, try compiling
+  with TCC and linking with GCC/Clang. If the linker has been
+  modified, try compiling with GCC/Clang and linking with TCC.
+
+- Test with ASan/UBSan to detect memory corruption and undefined behaviour:
+
+make clean
+./configure
+make
+make test
+cp libtcc.a libtcc.a.hide
+
+make clean
+./configure --extra-cflags="-fsanitize=address,undefined -g"
+make
+cp libtcc.a.hide libtcc.a
+make test
+
+  (On 64-bit architectures, misaligned access to struct CString is
+  expected.)
+
+- Test with Valgrind to detect some uses of uninitialised values:
+
+make clean
+./configure
+make
+# On Intel, because Valgrind does floating-point arithmetic differently:
+( cd tests && gcc -I.. tcctest.c && valgrind -q ./a.out > test.ref )
+make test TCC="valgrind -q --leak-check=full `pwd`/tcc -B`pwd` -I`pwd`"
+
+  (Because of how VLAs are implemented, invalid reads are expected
+  with 79_vla_continue.)