Add README (#6)

Closes #1

Author: migesok

README.md

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+# grpc-java-tools
+
+[![Build Status](https://github.com/evolution-gaming/grpc-java-tools/actions/workflows/ci.yml/badge.svg?branch=main)](https://github.com/evolution-gaming/grpc-java-tools/actions/workflows/ci.yml?query=branch%3Amain)
+[![Maven Central Version](https://img.shields.io/maven-central/v/com.evolution.jgrpc.tools/k8s-dns-name-resolver)](https://central.sonatype.com/artifact/com.evolution.jgrpc.tools/k8s-dns-name-resolver)
+
+A collection of Java libraries that supplement [grpc-java](https://github.com/grpc/grpc-java).
+
+The minimum supported JDK version is 17.
+
+## Table of Contents
+
+- [Modules](#modules)
+  - [k8s-dns-name-resolver](#k8s-dns-name-resolver)
+- [For Contributors](#for-contributors)
+
+## Modules
+
+### k8s-dns-name-resolver
+
+Provides a custom name resolver for working with headless Kubernetes services using DNS.
+
+Compared to the grpc-java built-in DNS name resolver, this one properly supports on-the-fly updates
+of the service pod list:
+
+- The built-in DNS name resolver relies on the JVM's DNS query logic and its cache.
+- If a new pod is added, it is not visible to the clients until a server closes one of the
+  connections, or the JVM DNS cache entry expires.
+- The JVM DNS cache (in JDK 17 and earlier, at least) does not honor DNS record TTL values and
+  instead uses a globally configured TTL for expiration.
+- So with the vanilla DNS name resolver, to discover a new pod quickly, you either need to set a
+  very low global JVM DNS TTL, or use a very low connection TTL on the servers.
+- Tampering with global JVM DNS settings might be undesirable because it could affect other
+  networking code, such as code that calls external HTTP services.
+- A low server connection TTL negates the benefits of long-lived HTTP/2 connections.
+
+`K8sDnsNameResolver` avoids these issues by using [dnsjava](https://github.com/dnsjava/dnsjava)
+to query DNS servers periodically (every 10 seconds by default) without using a DNS cache.
+
+It is useful if you cannot use a Kubernetes API-based or an xDS API-based resolver for your
+Kubernetes-deployed gRPC services.
+
+#### Getting Started
+
+Declare dependency on the library:
+[Maven Central](https://central.sonatype.com/artifact/com.evolution.jgrpc.tools/k8s-dns-name-resolver)
+
+`K8sDnsNameResolverProvider` is automatically discovered by the grpc-java runtime.
+
+You can then use the `k8s-dns` scheme in your gRPC client channel builder:
+
+```java
+ManagedChannel channel = NettyChannelBuilder
+    .forTarget("k8s-dns://my-svc.my-namespace.svc.my-cluster.local:8080")
+    .build();
+```
+
+`K8sDnsNameResolver` supports almost the same target URI format as the standard `DnsNameResolver`:
+
+- `k8s-dns://my-svc.my-namespace.svc.my-cluster.local` (default port)
+- `k8s-dns:///my-svc.my-namespace.svc.my-cluster.local` (default port)
+- `k8s-dns://my-svc.my-namespace.svc.my-cluster.local:8080`
+- `k8s-dns:///my-svc.my-namespace.svc.my-cluster.local:8080`
+
+## For Contributors
+
+### Build Requirements
+
+You will need:
+
+- JDK 17
+- [sbt](https://www.scala-sbt.org)
+
+The published library modules are pure Java, but some of the test code is written in Scala.
+
+### Useful SBT commands
+
+To reformat Java and Scala code run:
+
+```shell
+sbt fmt
+```
+
+Code formatting is verified by the build commands, and pull requests with incorrectly formatted code
+will not be accepted.
+
+A full build can be run with:
+
+```shell
+sbt build
+```