add readme to plugin-sdk subdir

packer-plugin-sdk/README.md

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+# Packer Plugin SDK
+
+This SDK enables building Packer plugins. This allows Packer's users to use both the officially-supported builders, provisioners, and post-processors, and custom in-house solutions.
+
+Packer itself is a tool for building identical machine images for multiple platforms from a single source configuration. You can find more about Packer on its [website](https://www.packer.io) and [its GitHub repository](https://github.com/hashicorp/packer).
+
+## Packer CLI Compatibility
+
+Packer v1.7.0 or later is needed for this SDK. Versions of Packer prior to that release are still compatible with third-party plugins, but the plugins should use the plugin tooling from inside earlier versions of Packer to ensure complete API compatibility.
+
+## Go Compatibility
+
+The Packer Plugin SDK is built in Go, and uses the [support policy](https://golang.org/doc/devel/release.html#policy) of Go as its support policy. The two latest major releases of Go are supported by the SDK.
+
+Currently, that means Go **1.14** or later must be used when building a provider with the SDK.
+
+## Getting Started
+
+See the [Extending Packer](https://www.packer.io/docs/extending) docs for a guided tour of plugin development.
+
+## Documentation
+
+See the [Extending Packer](https://www.packer.io/docs/extending) section on the Packer website.
+
+## Packer Scope (Plugins VS Core)
+
+### Packer Core
+
+ - acts as an RPC _client_
+ - interacts with the user
+ - parses (HCL/JSON) configuration
+ - manages build as whole, asks **plugin(s)** to manage the image lifecycle and modify the image being built.
+ - discovers **plugin(s)** and their versions per configuration
+ - manages **plugin** lifecycles (i.e. spins up & tears down plugin process)
+ - passes relevant parts of parsed (valid JSON/HCL) and interpolated configuration to **plugin(s)**
+
+### Packer Provider (via this SDK)
+
+ - acts as RPC _server_
+ - executes any domain-specific logic based on received parsed configuration. For builders this includes managing the vm lifecycle on a give hypervisor or cloud; for provisioners this involves calling the operation on the remote instance.
+ - tests domain-specific logic via provided acceptance test framework
+ - provides **Core** with template validation, artifact information, and information about whether the plugin process succeeded or failed.
+
+## Migrating to SDK from built-in SDK
+
+Migrating to the standalone SDK v1 is covered on the [Plugin SDK section](https://www.packer.io/docs/extend/plugin-sdk.html) of the website.
+
+## Versioning
+
+The Packer Plugin SDK is a [Go module](https://github.com/golang/go/wiki/Modules) versioned using [semantic versioning](https://semver.org/).
+
+## Contributing
+
+See [`.github/CONTRIBUTING.md`](https://github.com/hashicorp/packer-plugin-sdk/blob/master/.github/CONTRIBUTING.md)
+
+## License
+
+[Mozilla Public License v2.0](https://github.com/hashicorp/Packer-plugin-sdk/blob/master/LICENSE)