Issue #2762173 by megansanicki, artinst4, YesCT, mradcliffe, a_thakur,...

Issue #2762173 by megansanicki, artinst4, YesCT, mradcliffe, a_thakur, vermauv, cilefen, mathieso, pwolanin, mlhess: Add "composer install" step to install.txt file for when Drupal is downloaded using git