Add readme and license

LICENSE.md

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+Copyright (c) 2018-2022 Ruffle LLC <[email protected]> and Ruffle contributors
+                        (https://github.com/ruffle-rs/api-report/graphs/contributors)
+
+Permission is hereby granted, free of charge, to any
+person obtaining a copy of this software and associated
+documentation files (the "Software"), to deal in the
+Software without restriction, including without
+limitation the rights to use, copy, modify, merge,
+publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software
+is furnished to do so, subject to the following
+conditions:
+
+The above copyright notice and this permission notice
+shall be included in all copies or substantial portions
+of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
+ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
+TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
+PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT
+SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
+IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.

README.md

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+# API Report
+
+This is a tool used to generate a progress report from two given API states, a 'specification' and an 'implementation'.
+
+[Ruffle](https://github.com/ruffle-rs/ruffle/) uses this to create their automatic Flash AVM2 progress report,
+[as seen on their website](https://ruffle.rs/avm2.html).
+
+## Building from source
+
+[Follow the official guide](https://www.rust-lang.org/tools/install) to install Rust for your platform.
+
+Then simply `cargo build --release` to build the binary,
+or in the instructions below replace `avm_report` with `cargo run --` to run the tool directly.
+
+## Usage
+
+To see the full help info, run `avm_report --help`.
+
+You'll need two files, a `specification` that describes the API and an `implementation` that describes the current state
+of the API implemented. We provide an AVM2 specification in the repo for Ruffle's own use.
+
+Run `avm_report -s specification.json -i implementation.json -o report.json`.
+
+## Specification / Implementation format
+
+If you read Rust, there's a general view of the structure over at [src/specification.rs](src/specification.rs).
+
+## Report format
+
+This tool quanitifies progress with a points system:
+- Any "item" in `specification` is worth 1 point. If it's a function or a class or a field, doesn't matter. This is totalled in `max_points`.
+- If an item from `specification` exists in `implementation`, you gain a point in `impl_points`.
+- If that same item is marked as a "stub" in `implementation`, you gain a point in `stub_penalty`.
+
+To calculate a percentage, it's `impl_points / max_points` to see what exists, or `(impl_points - stub_penalty) / max_points` to see what's actually **done**.
+
+This allows you to show the stubs differently on a progress bar, for example.