Explain precompiled vignettes in contribution guide

[sbfnk]

          it should likely be added to the contributing guide if it isn't already (I think it isn't)

Originally posted by @seabbs in #458 (comment)

[jamesmbaazam]

I currently don't see where this would go based in the guide. I think the section on "bigger" changes can be structured as:

# Bigger changes
If you want to make a bigger change, it's a good idea to first file an issue and make sure 
someone from the team agrees that it’s needed. 
- If you’ve found a bug, please file an issue that illustrates the bug with a minimal [reprex] (https://www.tidyverse.org/help/#reprex) (this will also help you write a unit test, if needed).

+ ## New features & Bugs
+ If you have an idea for a feature or have found a bug, please file an issue. For bugs 
+ illustrate them with a minimal [reprex](https://www.tidyverse.org/help/#reprex)
+ (this will also help you write a unit test, if needed). 

+ ## Vignettes
+ If you have an idea for a vignette, please file an issue with an outline of the vignette 
+ to be discussed with the team first. Since the models in _EpiNow2_ have long run times 
+ in most cases, we precompile the vignettes before merging.  Please follow this guide 
+ on [how to precompute vignettes or pkgdown articles](https://ropensci.org/blog/2019/12/08/precompute-vignettes/).

[sbfnk]

That sounds good to me - would be great to have a PR for this.