doc: Write minimal usage

README.md

@@ -1,3 +1,48 @@
 # gazer
 
 gazer is my bumpversion tool (for training Rust programming).
+
+## Installation
+
+```
+cargo install --git https://github.com/attakei-lab/gazer.git
+```
+
+## Usage
+
+At first, you must generate configuration file -- `.gazer.toml`.
+You should edit it for your current version, because generated file is set `current_version = "0.0.0"`.
+
+```
+gazer init
+```
+
+If you want to up next version, run `gazer update|major|minor|patch`.
+
+* `update` accepts any semver.
+* `major` is shortcut to major version updating of semver.
+* `minor` is shortcut to minor version updating of semver.
+* `patch` is shortcut to patch version updating of semver.
+
+Example:
+
+```console
+# If current_version = "1.2.3"
+> gazer major  # Update to 2.0.0
+> gazer minor  # Update to 1.3.0
+> gazer patch  # Update to 1.2.4
+```
+
+## Configuration file
+
+### `current_version`
+
+Version value that gazer manages.
+
+### `files`
+
+List of replace target for update versioning.
+
+* `path`: File path of cwd.
+* `search`: Replace target line. `{{current_version}}` will be replaced for current version.
+* `replace`: Replaced text for target. `{{new_version}}` will be replaced for next version.