Add minimal documentation

Author: bensku

README.md

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+# TypeScript binding generator for Java
+java-ts-bind takes your Java source code and generates TypeScript types for it.
+It is meant to be used with [GraalJS](https://github.com/oracle/graaljs)
+to provide a strongly-typed scripting environment.
+
+This project was created for [CraftJS](https://github.com/Valtakausi/craftjs),
+a Bukkit plugin for writing plugins in JavaScript. It is based on earlier work
+by [Ap3teus](https://github.com/Ap3teus).
+
+No releases are currently provided. If you need it, compile it yourself
+(or open a bug in the issue tracker).
+
+## Usage
+This is a command-line application.
+
+* --format: output format
+  * Currently only TS_TYPES is supported
+* --in: input directory or source jar
+* --symbols: symbol sources (compiled jars)
+* --repo: Maven repo to fetch the source jar from
+* --artifact: Artifact to fetch from given repo
+  * tld.domain:artifact:version (Gradle-style)
+* --offset: path offset inside the input
+  * Mainly used for Java core types; see .github/workflows for an example
+* --include: prefixes for included paths
+  * By default, everything is included
+* --exclude: prefixes for excluded paths
+  * Processed after includes; nothing is excluded by default
+* --blacklist: blacklisted type fragments
+  * Types that have names which contain any of these are omitted
+  * Methods and fields that would use them are also omitted!
+* --packageJson: read these options from a JSON file
+  * The options should be placed under `tsbindOptions` object
+  * Names of options lack -- prefixes but are otherwise same
+  * Handy when you already have package.json for publishing
+
+## Limitations
+java-ts-bind does not necessarily generate *valid* TypeScript declarations.
+The results are good enough to allow strongly-typed scripts, but it is
+recommended that `noLibCheck` is used.
+
+Please also note that java-ts-bind provides *only the types*. Implementing
+a module loading system for importing them is left as an exercise for the
+reader. For pointers, see [CraftJS](https://github.com/Valtakausi/craftjs)
+which (at time of writing) implements a CommonJS module loader with
+Java and TypeScript.