Add convention for location of standard runfiles library.

hlopko@ approved this suggested convention before he left. It's consistent with what we already do in @bazel_tools.

RELNOTES: None
PiperOrigin-RevId: 261629882

site/docs/skylark/deploying.md

@@ -49,6 +49,9 @@ For example, suppose we are writing new rules for the (make-believe)
   mockascript/
     constraints/
       BUILD
+    runfiles/
+      BUILD
+      runfiles.mocs
     BUILD
     defs.bzl
   tests/
@@ -150,6 +153,13 @@ using [selects](https://docs.bazel.build/versions/master/be/functions.html#selec
 With custom constraints, you define a language that the whole Bazel ecosystem
 will speak.
 
+### Runfiles library
+
+If your rule provides a standard library for accessing runfiles, it should be
+in the form of a library target located at `//<LANG>/runfiles` (an abbreviation
+of `//<LANG>/runfiles:runfiles`). User targets that need to access their data
+dependencies will typically add this target to their `deps` attribute.
+
 ### Repository Rules
 
 #### Dependencies