Expand the README

Author: timholy

README.md

@@ -2,12 +2,12 @@
 
 CodeTracking is a minimal package designed to work with (a future version of)
 [Revise.jl](https://github.com/timholy/Revise.jl).
-Its main purpose is to allow packages that need to know the location
+Its main purpose is to support packages that need to know the location
 (file and line number) of code that might move around as it's edited.
 
 CodeTracking is a very lightweight dependency.
 
-Demo:
+Example:
 
 ```julia
 julia> using CodeTracking
@@ -16,5 +16,31 @@ julia> m = @which sum([1,2,3])
 sum(a::AbstractArray) in Base at reducedim.jl:648
 
 julia> file, line = whereis(m)
-("/home/tim/src/julia-1/usr/share/julia/base/reducedim.jl", 648)
+("/home/tim/src/julia-1/usr/share/julia/base/reducedim.jl", 642)
 ```
+
+In this (ficticious) example, `sum` moved because I deleted a few lines higher in the file;
+these didn't affect the functionality of `sum` (so we didn't need to redefine and recompile it),
+but it does change the starting line number of the file at which this method appears.
+
+## A few details
+
+CodeTracking won't do anything *useful* unless the user is also running Revise,
+because Revise will be responsible for updating CodeTracking's internal variables.
+(Using `whereis` as an example, CodeTracking will just return the
+file/line info in the method itself if Revise isn't running.)
+
+However, Revise is a fairly large (and fairly complex) package, and currently it's not
+easy to discover how to extract particular kinds of information from its internal storage.
+CodeTracking will be designed to be the new "query" part of Revise.jl.
+The aim is to have a very simple API that developers can learn in a few minutes and then
+incorporate into their own packages; its lightweight nature means that they potentially gain
+a lot of functionality without being forced to take a big hit in startup time.
+
+## Current state
+
+Currently this package is just a stub---it doesn't do anything useful,
+but neither should it hurt anything.
+Candidate users may wish to start `import`ing it and then file issues
+or submit PRs as they discover what kinds of functionality they need
+from CodeTracking.