Adds several annoying files to Unity3D ignore file, like platform cache files (Mac + Windows), the famous Unity3D pidb.meta

By having the leading `/` infront of the `*` makes these types of files relative to the directory the gitignore is in. If you create a Unity project inside of a git directory Unity forces you to create a new directory, thus not picking up these files.

Ignore the build directory in case a Unity build is initiated in the project.

Sysinfo.txt is generated by unity if occurs any crashes. The content consist of  users system information, running process etc. I don't think that should be shared with others.

Unity only generates project files in the top-level directory.
However, the current .gitignore will ignore all project files even
if they were not generated by Unity.

It is sometimes necessary to include non-Unity project files for
level editors or other utility projects that have been added to the
repository.

*.suo files are binary, user specific files generated by Visual Studio with some user configuration information, thus, don't need to be versioned.