README: improve Installation and Updating guides
README: improve Installation and Updating guides

Responding to user feedback... See: [Installation using wine gets stuck at .NET Framework 4.6.2](https://forum.winehq.org/viewtopic.php?f=8&t=31452).
Linux user's don't like to copy-paste random shell scripts from the Internet! An inexperienced Linux user will not think to remove a previously version of winetricks - so a manually installed and more up-to-date winetricks version may be ignored.

Updates to README:

* Breakdown the winetricks update script in excruciating detail. With helpful links to the TLDP website.
* Supply an alternative update script, to show how to set it up, using 'su' in place of 'sudo'.
* Don't use /usr/local/bin as an installation location, in case this isn't in a regular Linux user's default PATH.
* Reduce the complexity / usage of root privileges, for all scripts. Only escalate to root as required.
* Recommend to uninstall any distribution (package manager) version of winetricks first (with a stern warning!) Otherwise a manually installed version of winetricks will conflict with the packaged version. Single out Debian / Ubuntu - as the primary culprits; only offering de-installation instructions for apt.

Signed-off-by: Rob Walker <6d3cbd9beb33ded5964e4245f097c3042e5845a0@gmail.com>

Winetricks/winetricks,Winetricks/winetricks