Add Maven Central badge and update installation instructions

[sfloess]

Current State

README directs users to packagecloud.io. No Maven Central badge, and installation examples don't mention Central (even though v1.31 mentions Maven Central compatibility).

Proposed Changes

• Once published to Maven Central, add the standard Maven Central badge.
• Update the "Add to your project" section to show Maven Central as the primary source.

[sfloess]

Defer to #177 - Maven Central badge should be added AFTER library is published to Maven Central.

Badge will be:

[![Maven Central](https://img.shields.io/maven-central/v/org.flossware/jcommons.svg?label=Maven%20Central)](https://central.sonatype.com/artifact/org.flossware/jcommons)

Installation instructions can be added once published:

<dependency>
    <groupId>org.flossware</groupId>
    <artifactId>jcommons</artifactId>
    <version>1.31</version>
</dependency>

Keep open until #177 is complete.