2.3 release notes draft

doc/release-notes-v2-3.rst

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+=========================
+Duktape 2.3 release notes
+=========================
+
+Release overview
+================
+
+Main changes in this release (see RELEASES.rst for full details):
+
+* TBD.
+
+* DUK_USE_ALIGN_BY now always defaults to 8 (natural alignment) to avoid any
+  potentially unsafe assumptions about compiler behavior for unaligned memory
+  accesses and pointers (which may be an issue even on x86).
+
+Upgrading from Duktape 2.2
+==========================
+
+No action (other than recompiling) should be needed for most users to upgrade
+from Duktape v2.2.x.  Note the following:
+
+* TBD.
+
+* If you're working with a low memory target or any other target where memory
+  usage matters, you may want to force DUK_USE_ALIGN_BY to a lower value
+  (4 or 1) to reduce memory overhead.  For most targets the memory overhead
+  is not important.