Write README.

LICENSE

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 The MIT License (MIT)
 
 Copyright (c) 2023 ModOrganizer 2 Team

README.md

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+# Build with `mob` Github Action
+
+Action to build ModOrganizer 2 components within Github workflows.
+
+**Example usage**:
+
+```yml
+name: Build UI Base
+on:
+  push:
+    branches: master
+  pull_request:
+    types: [opened, synchronize, reopened]
+jobs:
+  build:
+    runs-on: windows-2022
+    steps:
+      - name: Build UI Base
+        uses: ModOrganizer2/build-with-mob-action@master
+        with:
+          mo2-third-parties: fmt gtest spdlog boost
+          mo2-dependencies: cmake_common
+
+```
+
+The action must be run on `windows-2022` since MSVC is required.
+
+## Configuration
+
+The following options should be specified most of the time:
+
+- `mo2-third-parties` - List of project to build with `mob` before any `super` task,
+  e.g., `fmt`, `spdlog`, `boost`, `pybind11`, etc.
+  - these dependencies are cached between runs depending on the latest release of `mob`.
+- `mo2-dependencies` - List of MO2 components to build before the current one.
+  - these components will be built using `mob` and the current branch (when possible),
+    e.g., if the current branch is `dev-branch`, components will be built on the
+    `dev-branch` branch (if available) or `master` otherwise.
+  - these builds are not cached between runs.
+
+The following options are not mandatory but can be used to customize the build:
+
+- `mo2-branch` - The branch to build, default to the current branch.
+- `mo2-cmake-command` - The CMake command to run with `mob cmake`, default to `..`
+  (same as `mob`).
+- `qt-install` - Default is `'true'`, set to `'false'` to disable installing Qt (build
+  will most likely fail unless Qt is installed before running this action).
+- `qt-version` - Version of Qt to install. Default is to use the version defined by
+  `mob.ini` in the `mob` repository.
+- `qt-modules` - List of extra Qt modules to install.