Git-Branches.md

+The OpenAPI Generator project works on multiple versions in parallel:
+
+* upcoming patch release
+* next minor release containing breaking changes with fallback
+* next major release containing breaking changes with no fallback
+
+## Branches
+
+To support several versions, several branches are used:
+
+<img width="800" alt="openapi-generator_branches" src="https://user-images.githubusercontent.com/1222165/41271120-03e688c6-6e0f-11e8-80fb-f1616c05aa04.png">
+
+
+Status in June 2018:
+
+* [`master`](https://github.com/OpenAPITools/openapi-generator/tree/master) branch is for upcoming patch releases (`3.0.1`, `3.0.2`, `3.0.3`, ...)
+* [`3.1.x`](https://github.com/OpenAPITools/openapi-generator/tree/3.1.x) branch for the next minor release
+* [`4.0.x`](https://github.com/OpenAPITools/openapi-generator/tree/4.0.x) branch for the next major release
+
+
+### Merge direction
+
+To port changes from one branch to an other, the merge direction is always from the lowest version to the newer.
+
+Example:
+
+* Merge `master` branch into `3.1.0` branch
+* Merge `3.1.0` branch into `4.0.0` branch
+
+Those merge commits are done regularly by the core team.
+
+Merge commit is kept in the history in order to facilitate merge conflict resolution.
+
+
+### Merging back on master
+
+When we drop the support for `3.0.x`, we'll merge `3.1.0` back into `master`.
+
+
+## Solving conflicts during merge
+
+> TODO, add useful git command, inspired by the discussion in https://github.com/OpenAPITools/openapi-generator/issues/245
\ No newline at end of file