Add changelog entry for the janestreet camlp4 split

CHANGES.md

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+## Camlp4 syntax extensions split from Jane Street packages (2016-01-11)
+
+Jane Street packages no longer support camlp4 after the 113.09.00
+series.
+
+For Jane Street packages that contain both a library and a camlp4
+syntax extension, the latter is extracted to its own package.
+
+The following table summarize the changes:
+
+Package     | Old findlib name of syntax | New camlp4-only package | version
+------------|----------------------------|-------------------------|----------
+sexplib     | sexplib.syntax             | pa\_sexp\_conv          | 113.00.00
+bin\_prot   | bin\_prot.syntax           | pa\_bin\_prot           | 113.00.00
+typerep     | typerep.syntax             | pa\_typerep\_conv       | 113.00.00
+fieldslib   | fieldslib.syntax           | pa\_fields\_conv        | 113.00.00
+variantslib | variantslib.syntax         | pa\_variants\_conv      | 109.15.03
+
+### Upgrading
+
+For the long term it is recommended to switch to the corresponding ppx
+alternative (replace the pa\_ prefix by ppx\_ to get the ppx package
+name).
+
+Until then packages that are using one of the camlp4 syntax extension
+must upgrade their build system to refer to the new package
+name. Simply replace the name in the second column by the one in the
+third column.
+
+Running the following shell command on the sources should do:
+
+    sed -ri 's/\<sexplib\.syntax\>/pa_sexp_conv/g;
+             s/\<bin_prot\.syntax\>/pa_bin_prot/g;
+             s/\<typerep\.syntax\>/pa_typerep_conv/g;
+             s/\<fieldslib\.syntax\>/pa_fields_conv/g;
+             s/\<variantslib\.syntax\>/pa_variants_conv/g' \
+      `ag -l .`
+
+The new dependency must also be added to the opam file.
+
+### Automatic upgrade of the opam-repository
+
+To avoid the opam-repository being in a inconsistent state once the
+camlp4-free Jane Street packages are released, the following automatic
+change is applied to the
+[opam repository](https://github.com/ocaml/opam-repository):
+
+```
+for js_pkg in "sexplib" "bin_prot" "type_conv" "variantslib" "fieldslib":
+  for pkg in all_packages():
+    if pkg depends on js_pkg and "type_conv":
+      pkg.add_constraint(js_pkg < next-version-of(js_pkg))
+```
+
+This is an over-approximation as some package might not use the syntax
+extension at all.
+
+Following this automatic change maintainers should:
+
+- just remove the constraint if the package doesn't use the syntax extension
+- do a new release that depend on the new camlp4-only package