update readme

README.md

 # Installation
-To install this package directly from git, you will need the devtools package, installable by running:
+To install this package directly from git, you will first need the devtools and getPass packages if not already installed:
 ```
-install.packages("devtools")
+install.packages(c("devtools", "getPass"))
 ```
 
+Then, the ASPREE package itself can be installed using the following command:
+```
+devtools::install_git("https://gitlab.erc.monash.edu.au/aspree/aspree-r-package.git",
+    credentials = git2r::cred_user_pass(readline(prompt="USERNAME: "),
+    getPass::getPass()))
+```
+You will be prompted for your Monash GitLab credentials before installation proceeds.