Contributing guide: tag message instructions

[DanicaSTFC]

• "The should be taken from the changelog (below)." should be changed to
  "The should be a more concise text taken from the changelog (below)."

• Adding also "Refer to changelog for more details."

• If there are backwards incompatible features, they should be explained

• Itemise messages eg.

• Adds the class AdvancedDialogForm.

• Adds methods to insert widgets and remove widgets in the forms.

• Adds addSpanningWidget method to FormDockWidget.

• Adds user-defined functionalities for Ok and Cancel buttons in FormDialog.

• Adds several unit tests and examples.

• Deprecates getWidgetStates to be getSavedWidgetStates.

• Removes _addWidget.

• Simplifies conda environment configuration for development.

• Add instructions to push tag:
  git push origin tag <tag_name>

• add conda-forge will crawl https://pypi.org/project/eqt/ regularly & release it by itself

• add link to https://github.com/conda-forge/eqt-feedstock

• do not use `` as they will not render properly in github