[doc] add RELEASING.md

RELEASING.md

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+
+### Versioning
+
+Version = MAJOR (>=1 year), MINOR (>= 1 month), PATCH
+
+Version Status = Dev / Release
+
+
+### Git
+
+Branches
+    develop: active development branch
+    master: latest release
+    vMAJOR.MINOR: release branch
+
+Tags
+    vMAJOR.MINOR.PATCH: tag for each release
+
+
+### Release Process
+
+ - Check if we need to upgrade some dependencies
+   - CMake all-in-one
+     - CMakeLists.txt in the root folder (use docker/check_dependencies_releases.py)
+     - Push updates to https://hub.docker.com/r/alicevision/alicevision-deps/tags
+   - Check vcpkg
+     - Update https://github.com/alicevision/vcpkg branch: alicevision_master
+     - Upload result on gdrive and update INSTALL.md
+   - Update INSTALL.md
+     - Update minimal versions if needed
+     - Update vcpkg install command if needed
+ - Source code
+   - Create branch from develop: "rcMAJOR.MINOR"
+   - Modify version in code, version status to RELEASE (src/aliceVision/version.hpp)
+   - 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 & update pre-compiled dependencies on gdrive if needed
+ - 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 DEV
+   - PR to develop: "Start Development MAJOR.MINOR"
+
+
+### 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 (src/aliceVision/version.hpp)
+   - Update release note
+ - Build step
+ - Uploads
+ - Github release note
+