update README

README.md

@@ -2,23 +2,22 @@
 
 # asdf-sync [![Build](https://github.com/robzr/asdf-sync/actions/workflows/build.yml/badge.svg)](https://github.com/robzr/asdf-sync/actions/workflows/build.yml) [![Lint](https://github.com/robzr/asdf-sync/actions/workflows/lint.yml/badge.svg)](https://github.com/robzr/asdf-sync/actions/workflows/lint.yml)
 
-[sync](https://github.com/robzr/asdf-sync) plugin for the [asdf version manager](https://asdf-vm.com).
+[sync](https://github.com/robzr/asdf-sync) plugin for the [asdf version manager](https://asdf-vm.com) providing Semantic Versioning constraint support and more.
 
 </div>
 
 # Contents
 
 - [Dependencies](#dependencies)
 - [Install](#install)
+- [Usage](#usage)
+- [Credits](#credits)
 - [Contributing](#contributing)
 - [License](#license)
 
 # Dependencies
 
-**TODO: adapt this section**
-
-- `bash`, `curl`, `tar`, and [POSIX utilities](https://pubs.opengroup.org/onlinepubs/9699919799/idx/utilities.html).
-- `SOME_ENV_VAR`: set this environment variable in your shell config to load the correct version of tool x.
+If `asdf` works, so does this. All it needs is Bash v3 or newer.
 
 # Install
 
@@ -33,21 +32,84 @@ asdf plugin add sync https://github.com/robzr/asdf-sync.git
 sync:
 
 ```shell
-# Show all installable versions
-asdf list-all sync
+# asdf-sync has hooks to run via asdf
+asdf sync help
+```
+
+# Usage
 
-# Install specific version
-asdf install sync latest
+asdf-sync extends the functionality of [asdf](https://github.com/asdf-vm/asdf),
+using the standard syntax [.tool-versions asdf config file](https://asdf-vm.com/manage/configuration.html#tool-versions),
+with optional additional configuration embedeed in specially formatted comments
+to provides the following functionality:
+- automatically installs and updates any plugins by interpreting .tool-versions syntax
+- can read standard format version constraints embeded in .tool-verisons comments
+- can auto-update .tool-versions file, effectively using it as a version lockfile
+- can read and use custom plugin URLs embedded in .tool-version comments
 
-# Set a version globally (on your ~/.tool-versions file)
-asdf global sync latest
+## Configuration
 
-# Now sync commands are available
-asdf-sync --help
+A standard syntax .tool-versions file is all that is needed for basic asdf-sync
+functionality.
+```
+sver 1.2.0
+terraform 1.5.7
+terragrunt 0.51.5
+```
+Now, running `asdf sync` will ensure these plugins are installed.
+
+## Version Constraint Interpretation
+
+asdf is great, but there is no native format for intelligent version constraints.
+Version constraints can be a power mechanism to enable automatic updates while
+preventing breaking changes. If a program is versioned in accordance with
+[Semantic Versioning](https://semver.org), it should be safe to simply follow
+updates within the same major version, as SemVer specifies that breaking changes
+necessitate a major version increase.
+```
+sver 1.2.0        # sync-constraint="~> v1.2, !pre"
+terraform 1.5.7   # sync-constraint="~> v1.5, !pre"
+terragrunt 0.51.5 # sync-constraint="~> v0.51.5, !pre"
+```
+Using a `sync-constraint="<constraint>"` token in .tool-versions will assign a
+version constraint to a given package. Upon running `asdf sync`, the latest
+versions matching the constraint will be determined, the .tool-versions file
+will be updated accordingly, and the packages will automatically be installed.
+
+## asdf Plugin URLs
+
+If a plugin is not in [asdf plugin registry](https://github.com/asdf-vm/asdf-plugins),
+the only way to install it is to manually specify the URL. Anyone running asdf
+out of CI/CD, for example, will need to manually manage these plugin URLs,
+and run `asdf plugin add <name> <url>` for each plugin. By using another token
+embedded in .tool-versions comments, asdf-sync can take care of this
+automatically.
 ```
+sver 1.2.0        # sync-url=https://github.com/robzr/asdf-sver
+terraform 1.5.7   # sync-constraint="~> v1.5, !pre" sync-url=https://github.com/blah/blah
+terragrunt 0.51.5 # 
+```
+
+## asdf Plugin refs
+
+Although not yet supported by asdf, the `sync-ref=<ref>` token can also be
+used to override a default plugin git ref.
+```
+sver 1.2.0        # sync-url=https://github.com/robzr/asdf-sver sver-ref=development
+terraform 1.5.7   # sync-constraint="~> v1.5, !pre" sync-url=https://github.com/blah/blah
+terragrunt 0.51.5 # 
+```
+
+## Help!
+
+At any time, just run `asdf sync help` for detailed usage instructions.
+
+# Credits
 
-Check [asdf](https://github.com/asdf-vm/asdf) readme for more instructions on how to
-install & manage versions.
+- asdf-sync was inspired by Nicholas Christopher's
+[syncher](https://github.com/nwillc/syncher/tree/master) program.
+- My [sver](https://github.com/robzr/sver) embeddable Semantic Version utility
+library is embedded in, and used by asdf-sync.
 
 # Contributing