Add new file about how to make a release

doc/DEV.md

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+How to make a release?
+======================
+
+1) Check that all test pass on all primary systems.
+   Run 'make check' on all primary systems.
+   No error shall be reported.
+   
+2) Check that no memory leak is reported by valgrind:
+   Run 'make valgrind' in tests folder on a system.
+
+3) Check that no undefined behavior, nor data race, ... is reported by sanitizer:
+   Run 'make sanitize'  in tests folder on a system.
+   This needs at least GCC 7
+
+4) Check that no new warning is reported by static analyser:
+   Run 'make scan-build' in tests folder on a system.
+   This needs at least clang 4.
+   Currently there is a false positive detected in m-shared:
+   "Argument to free() is offset by 8 bytes from the start of memory allocated by malloc()"
+   scan-build is not able to properly track that this case cannot open.
+   
+5) Check that coverage is reasonnable.
+   Run 'make coverage' and analyze result.
+
+In case of problem, open a problem error.
+
+List of primary systems
+=======================
+
+* gcc 4.9 on linux/x86-64
+* gcc 7 on linux/x86-64
+* clang 3.5 on linux/x86-64
+* clang 4 on linux/x86-64
+* g++ 7 on linux/x86-64
+* tcc on linux/x86-64