Updated Linux build instructions.

[AlbertoCasasOrtiz]

Updated linux build script.

Fixes issue #1375

Brief summary of changes

Added new build script for Linux (Ubuntu 18.04).

Testing I've completed

Executed build script on a fresh Ubuntu 18.04 installation.

CHANGELOG.md (choose one)

• no need to update because this is just a change in the readme file.

Known related issues

• After executing the INSTALL script, opensim-gui is not appearing in Ubuntu's applications menu. The issue OpenSim not appearing in Ubuntu applications menu #1373 describes this error and there is a pull request with a proposed solution to this: Fix installer ubuntu #1374
• When you execute opensim-gui, when it is loading, there is a window indicating that libadolc.so.2 could not be found. This is a known issue reported here (where there is also a workaround) and there is a pull request in the opensim-core repository working on solving it: #2930