Changes to Maven POM file in order to publish it to Maven Central

Modified the project's Maven POM file for the following:

1. Ensuring that it contains all the necessary information for the project to be accepted into the Maven Central repository (repo.maven.org / repo1.maven.org / search.maven.org);
2. Added Maven GPG plugin to enable GnuPG signing of the project artifacts, which is a mandatory step for project artifacts to be accepted into the Maven Central repository;
3. Added instructions on uploading the project to the Maven Central repository in the readme file.

I have pushed the first version of the project to Maven Central.  It can be included in Maven POM files as:

```
<dependency>
    <groupId>org.owasp</groupId>
    <artifactId>csrfguard</artifactId>
    <version>3.0.0</version>
</dependency>
```