feat: update changelog (#131)

closes #130 

- CHANGELOG.md file for people who can view changelog from github
---
`iris changelog` command:
`iris changelog [version]` (e.g. iris changelog v0.5.1)
`iris changelog -n 3` (see changelog of 3 latest releaseas)

=> `iris changelog` looks just like CHANGELOG.md, grouped headers and
all. It only stops looking that way if we personally overwrite that
release's description with something else
=> With 1-hour local cache (invalidated on updater-channel change),
`--refresh` flag to bypass, graceful fallback to stale cache on GitHub
rate-limiting

---
- New auto update config
```toml
# 0 = off (default, notify only), 1 = auto-install, 2 = always confirm first
auto-update = 1
```

- New just debug-changelog and just debug-autoupdate recipes (fully
offline, mocked HTTP servers, no real network/config touched)

README.md

@@ -229,6 +229,21 @@ cd iris
 just reload
 ```
 
+## Update
+
+```bash
+iris update
+```
+
+Checks for and installs the latest release. To see what's changed first:
+
+```bash
+iris changelog       # latest release
+iris changelog -n 3  # last 3 releases
+```
+
+IRIS can also check for and install updates on its own - see `updater.auto-update` in the [configuration guide](#configuration-guide) below.
+
 ## Uninstall
 
 To completely uninstall IRIS, remove all configurations, and clean up your shell integration files, simply run:
@@ -310,6 +325,7 @@ deduplicate-branches = true
 check-on-startup = true
 channel = "stable" # "stable" or "nightly"
 check-interval = "24h"
+auto-update = 0 # 0 = off, 1 = auto-install, 2 = always confirm first
 
 [ai]
 enabled = false