chore: added README.md

Author: max-prosper

README.md

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+# Kafka Connect SMT to convert values to JSON strings
+This lib implements Kafka connect SMT (Single Message Transformation) to
+convert values of specified fields to JSON strings (stringify).
+
+## Config
+Use it in connector config file like this:
+~~~json
+...
+"transforms": "stringify",
+"transforms.stringify.type": "com.github.maxprosper.smt.stringifyjson.StringifyJson$Value",
+"transforms.stringify.targetFields": "field1,field2",
+...
+~~~
+
+Use dot notation for deeper fields (e. g. `level1.level2`).
+
+## Install to Kafka Connect
+After build copy file `target/stirngify-json-smt-0.0.0-jar-with-deps.jar`
+to Kafka Connect container `` copying to its docker image or so.
+
+It can be done adding this line to Dockerfile:
+~~~Dockerfile
+COPY ./target/stringify-json-smt-0.0.0-jar-with-deps.jar $KAFKA_CONNECT_PLUGINS_DIR
+~~~
+
+Or download current release:
+~~~Dockerfile
+RUN curl -fSL -o /tmp/plugin.tar.gz \
+    https://github.com/max-prosper/stringify-json-smt/releases/download/0.0.0/stringify-json-smt-0.0.0.tar.gz && \
+    tar -xzf /tmp/plugin.tar.gz -C $KAFKA_CONNECT_PLUGINS_DIR && \
+    rm -f /tmp/plugin.tar.gz;
+~~~
+
+## Build release file
+- Increment version in `pom.xml` (e.g. to `0.0.1`).
+- Run build script: `./scripts/build-release.sh 0.0.1`.
+- Take `*.tar.gz` file from `target` folder and publish it.