skan.csr: Clarify that a skeleton's distances list is initially empty

[evan10s]

While working on a project using this library (which has been super helpful!), I noticed that the distances array for my skeleton seemed to contain very small values. Looking at the code, it appears that distances is not initialized until the path_lengths method is called on a skeleton. This is just a small change to indicate this fact in the docs to reduce confusion in the future.

[coveralls]

Coverage remained the same at 99.453% when pulling ff8cff0 on evan10s:master into ef6df1f on jni:master.

[jni]

Whoa! Thanks for spotting this! Long term we probably want to fix this properly (e.g. distances could become a cached property), but this improves things for now.

Glad you're finding it useful!