Improve README.md

[alexander-rass]

• Download link to github repository
• links to configuration files of examples and other files within the repository
• link to images within the repository instead of duplicating it for gh-pages
• avoid line breaks within formulas

[alexander-rass]

• Download link is introduced
• links to various files are activated
• avoiding line breaks within formulas not successful as nobr tag seems to be not supported by doxygen and github using markdown. nobr tag is removed from github showing the README.md. Therefore I removed the tags of this kind and additional handling to cover doxygen needs.
• some minor improvements in the README.md
• Renaming LICENSE.md to LICENSE (withoud ending md) to fix an issue of doxygen which leads to the effect that the link to the license file can not be introduced