Provide basic info about packages

[lgatto]

A short intro/section about packages in the R intro lesson, with longer and more formal sections (including different sources to install packages from) in @kevinrue.

[lgatto]

Objective of this course is for students to be able to

1. install packages
2. load packages
3. recognise when a package isn't loaded/installed

In the next steps section, when talking about Bioconductor, mention and install with BiocManager::install().

[lgatto]

TODO: make sure to use install.packages() through the lesson.

[csoneson]

I guess this is implicit in the list above, but just to add

4. Have some idea about the difference between installing and loading (e.g., you don't have to run install.packages() in every new session).