Add distrib doc

DISTRIBUTION.md

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+## Indications for the preparation of successive releases
+
+The branch `coq-master` is monitored by Coq continuous integration, so that
+the Coq developers help making sure that the code of this Coq plugin is
+compatible with the master branch of the Coq development.
+
+Every Coq release, it is necessary to produce a tagged version of this package
+so that this version can then be distributed with coq-platform.
+
+There are only few operations to perform, thanks to the coq-community templates.
+
+ - First create a branch for this new release.  For instance, for version 8.14
+   of Coq, the branch `coq-v8.14` was created.
+
+ - Edit file `meta.yml`  To know what modifications should be performed, the
+   best is to compare versions of `meta.yml` for branch `coq-master` and the
+   latest branch for a coq release.
+
+ - To make sure the modifications of `meta.yml` are taken into
+   account, the command `generate.sh` from `coq-community/templates`
+   should be run.
+
+ - Edit file `configure.ac` Here again, the modifications should be
+   similar to modification from branch `coq-master` to the latest
+   branch for a coq release.
+
+ - The new version of `meta.yml`, `configure.ac` and the files
+   generated by `generate.sh` that are under version control should
+   then be committed on this branch.  This branch should be pushed on github.
+
+ - run the following commands:
+ ```bash
+   autoconf
+   ./configure
+   make distrib
+```
+
+   This creates a tar-gz archive with the right naming.
+
+ - on github, create a new release for this version.  The name is
+   usually composed of two parts.  The first part is the revision for
+   coq-dpdgraph (as used in the `coq-master` version of
+   `configure.ac`), then the + sign, then the targeted revision of
+   coq.  The contents of this release are given by the branch you created.
+
+   The archive that was generated by `make distrib` should be added to
+   that release (as an asset).
+
+ - The next step is to add an opam file for this revision in
+   `opam-coq-archive`.
+
+ - Clone the `opam-coq-archive` repository, create a working branch
+   stemming from `master`, and then add the new opam file at the
+   relevant place.
+
+ - If the targeted revision of Coq is already
+   official, then this opam file can be added directly in the
+   `released` directory.  The name of the directory for this file
+   should be the similar to the name of the tag
+   (`v` being replaced by `coq-dpdgraph.`).  The opam file should be
+   inspired from the opam file for the previous revision except that
+   the url src and checksum should be adapted to the new name and its
+   checksum (obtained using `sha512sum`.
+
+ - If you want to test that your new `opam` file is correct, a first
+   step is to call `opam lint` on this file.  The second step is to
+   test an installation.  Here is a way to do it quickly:
+
+```bash
+      opam repo add tmp-coq-released .../opam-coq-archive/released
+      opam install coq-dpdgraph
+```
+
+   Do not forget to perform an `opam update` after every modification
+   to any file in the opam archive.