Indicate that the create_package parentDirectory should not be an existing R project

[navonacalarco]

The Making Packages episode references an abstract parentDirectory, in which one should create the package 'tempConvert'.

I think it would be helpful to explicitly indicate that this new parentDirectory ought not be nested within the r-novice-inflammation directory, previously created as an 'R project' in the first episode (and I believe the working directory assumed by all prior episodes). The reason is that the create_package function throws a warning if attempting to create a new project within an existing project, as it is "rarely a good idea".