Add README

README.md

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-# jabel
+# Jabel - Javac 12 plugin that makes it emit Java 8 bytecode
+
+> Because life is too short to wait for your users to upgrade their Java!
+
+
+## Motivation
+
+While Java is evolving and introduces new language features, the majority of OSS libraries
+are  still using Java 8 as their target because it still dominates.
+
+But, since most of features after Java 8 did not require a change in the bytecode,
+`javac` could emit Java 8 bytecode even when compiling Java 12 sources.
+
+## How Jabel works
+
+Although Jabel is an annotation processor, it does not run any processing,
+but instruments the java compiler classes and makes it think that Java 12 target
+does not support certain bytecode features like indy strings, nestmates, and others.
+
+It also makes it emit Java 8 bytecode major/minor version (instead of Java 12 experimental).
+
+The result is a valid Java 8 bytecode for your switch expressions, `var` declarations,
+and other features unavaliable in Java 8.
+
+## How to use
+
+TODO