Reference updatesite made by GitHub Action in Pull Request template

[AObuchow]

As mentioned by @ingomohr here, it'd be nice to mention to contributors who submit a PR that their fork will automatically create an updatesite if they are working on their forks master branch.

This could be nice for contributors who aren't familiar with Maven or Eclipse plugin development.

The Pull Request template would have to make special mention that the contributors name on GitHub must be used to obtain the correct updatesite, eg: https://raw.githubusercontent.com/[GitHub Username]/Eclipse-Modern-Dark-Theme/updatesite/updatesite/

[AObuchow]

Now that #90 is merged, a contributor's fork will automatically build an updatesite for each branch the make (on their fork). The contribution guidelines should describe how to determine the updatesite URL for their fork's branch.