Take care of Nuxeo Server Readme.html/.txt

There are 2 README files in the Nuxeo Server itself:

• https://github.com/nuxeo/nuxeo/blob/master/nuxeo-distribution/nuxeo-server-tomcat/src/main/resources/tomcat/readme.html
• https://github.com/nuxeo/nuxeo/blob/master/nuxeo-distribution/nuxeo-server-tomcat/src/main/resources/tomcat/README.txt

Update and keep the README.txt file, renamed to README.md.
Remove the readme.html file.
Update the server assembly.xml to move the following files to doc-tomcat:

• BUILDING.txt
• CONTRIBUTING.md
• README.md (the one from Tomcat)