Added RELEASE_PROCESS document

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+## Definition
+
+A release is mostly a git tag of http://github.com/wallabag/wallabag, following [semantic versioning](http://semver.org).
+The last release at the time of writing is 2.0.0-alpha.2, from the v2 branch.
+
+### Steps
+- Update `wallabag.version` is up-to-date in `app/config/config.yml` if necessary
+- run composer update to make sure `composer.lock` is up-to-date
+- add and update `composer.lock`: `git add -f composer.lock && git commit -m "Added composer.lock for 2.0.0-alpha.3 release"`
+- create the tag: `git tag 2.0.0-alpha.3`
+- remove composer.lock, and commit: `git rm composer.lock && git commit -m "Removed composer.lock"`
+- push the tag: `git push origin 2.0.0-alpha.3`
+- go to http://github.com/wallabag/wallabag/releases
+- find the tag that was created in the list, click on the tag. Edit the release name / description
+
+### `composer.lock`
+A release tag must contain a `composer.lock` file. It sets which dependencies were available at the time a release was done,
+making it easier to fix issues after the release. It also speeds up `composer install` on stable versions a LOT, by skipping the
+dependencies resolution part.
+
+Since `composer.lock` is ignored by default, either it must be removed from `.gitignore` _in the release branch_,
+or it must be added using `git add --force composer.lock`.
+
+### Target PHP version
+`composer.lock` is _always_ built for a particular version, by default the one it is generated (with `composer update`).
+
+If the PHP version used to generate the .lock isn't a widely available one (like PHP 7), a more common one should
+be locally specified in `composer.lock`:
+
+```json
+    "config": {
+        "platform": {
+            "php": "5.5.9",
+            "ext-something": "4.0"
+        }
+    }
+```