Add README file for mvn plugin

Signed-off-by: Marc Cornellà <marc.cornella@live.com>

plugins/mvn/README.md

+## The mvn Plugin
+
+The [mvn](https://github.com/robbyrussell/oh-my-zsh/tree/master/plugins/mvn) plugin provides many [aliases](#Aliases).
+
+Enable it by adding _mvn_ to the [_plugins array_](https://github.com/robbyrussell/oh-my-zsh/blob/master/templates/zshrc.zsh-template#L48) before sourcing OMZ (see [Plugins](https://github.com/robbyrussell/oh-my-zsh/wiki/Plugins)).
+
+## Aliases
+
+| Alias                | Command                                                                                                                                   |
+|:---------------------|:------------------------------------------------------------------------------------------------------------------------------------------|
+|mvncie|mvn clean install eclipse:eclipse|
+|mvnci|mvn clean install|
+|mvncist|mvn clean install -DskipTests|
+|mvne|mvn eclipse:eclipse|
+|mvnd|mvn deploy|
+|mvnp|mvn package|
+|mvnc|mvn clean|
+|mvncom|mvn compile|
+|mvnct|mvn clean test|
+|mvnt|mvn test|
+|mvnag|mvn archetype:generate|
+|mvn-updates|mvn versions:display-dependency-updates|
+|mvntc7|mvn tomcat7:run|
+|mvnjetty|mvn jetty:run|
+|mvndt|mvn dependency:tree|
+|mvns|mvn site|
+|mvnsrc|mvn dependency:sources|
+|mvndocs|mvn dependency:resolve -Dclassifier=javadoc|