Enhance documentation for ignoring (alpha|beta)

[onacit]

I just found that the following example documented on Version number rules may be enhanced.

  <version>.+-(alpha|beta)</version>

Since any alpha or beta releases may has additional sequence number after the word, it would be nice the example show like,

  <version>.+-(alpha|beta).+</version>

Thanks.

[slawekjaranowski]

PR is welcome ... simply edit: https://github.com/mojohaus/versions/blob/master/versions-maven-plugin/src/site/markdown/version-rules.md.vm

output can be checked by executing mvn site site:stage and look into target/staging

[onacit]

@slawekjaranowski As you wish, Milord.
#1145