Add reminder about installing dependencies.

[hlapp]

This may seem trivial to an experienced R programmer, but the default is in fact not to install dependencies. If one forgets to enable that, errors will result later that can be obscure to trace
back to the failure of installing dependencies earlier.

[cboettig]

And yet again you've encountered an error that is more tricky than it may appear!

Since writing this paper, odesolve has been deprecated on CRAN in favor of deSolve, whose lsoda function does not in fact use the same function call! Because of the branch becoming repo problem I discuss in #2, this change was fixed on all branches of the earlywarning package but never ported here. (which is why this package should have been a new package depending directly on earlywarning.) It is of course an annoying deeper issue that package dependencies can change on CRAN in this way, either by a package vanishing, or worse, just by it's API changing in some update (CRAN doesn't enforce backwards compatible function definitions, and many scientist R developers don't even give that a thought, since many imagine only interactive use of their functions rather than scripts and dependencies).

So I should merge the desolve changes from the earlywarning branch, and update the README and DESCRIPTION accordingly...

@hlapp If you haven't seen it, your experiences in this exercise might be very relevant to the Mozilla Science Lab / Software Carpentry discussion on code reuse, see mozillascience/code-research-object#2 There's a desire to go beyond Publish your code, it's good enough but also recognize the lack of incentive to do this 3x extra work of making research code easily deploy-able and reusable.

[cboettig]

Done. Most of this should now be handled well by devtools. Added note to readme about installing populationdynamics too.