[doc] describe releasing process

RELEASING.md

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+
+### Versioning
+
+Version = MAJOR (>=1 year), MINOR (>= 1 month), PATCH
+
+Version Status = Develop / Release
+
+
+### Git
+
+Branches
+    develop: active development branch
+    master: latest release
+    vMAJOR.MINOR: release branch
+
+Tags
+    vMAJOR.MINOR.PATCH: tag for each release
+
+
+### Release Process
+
+ - Prepare the AliceVision release: https://github.com/alicevision/AliceVision
+ - Update INSTALL.md and requirements.txt if needed
+ - Source code
+   - Create branch from develop: "rcMAJOR.MINOR"
+   - Modify version in code, version status to RELEASE (meshroom/__init__.py)
+   - Create Release note (using https://github.com/cbentejac/github-generate-release-note)
+   - PR to develop: "Release MAJOR.MINOR"
+ - Build
+   - Build docker & push to dockerhub
+   - Build windows
+ - Git
+   - Merge "rcMAJOR.MINOR" into "develop"
+   - Push "develop" into "master"
+   - Create branch: vMAJOR.MINOR
+   - Create tag: vMAJOR.MINOR.PATCH
+   - Create branch from develop: "startMAJOR.MINOR"
+ - Upload binaries on fosshub
+ - Fill up Github release note
+ - Prepare "develop" for new developments
+   - Upgrade MINOR and reset version status to Develop
+   - PR to develop: "Start Development MAJOR.MINOR"
+ - Communication
+   - Email on mailing-list: alicevision@googlegroups.com
+   - Message on linkedin: https://www.linkedin.com/groups/13573776
+   - Message on twitter: https://twitter.com/alicevision_org
+
+### Upgrade a Release with a PATCH version
+
+ - Source code
+   - Create branch from rcMAJOR.MINOR: "rcMAJOR.MINOR.PATCH"
+   - Cherry-pick specific commits or rebase required PR
+   - Modify version in code
+   - Update release note
+ - Build step
+ - Uploads
+ - Github release note
+ - Email on mailing-list
+