Explain why we are not using 'cache: cargo' with Travis

diff --git a/.travis.yml b/.travis.yml
index e8c14d24a6664d4c143a8aada1d5567cdc0e264c..6e7767febe6408d1a310f1aca55b8ba99e1ffdce 100644 (file)
--- a/.travis.yml
+++ b/.travis.yml
@@ -6,6 +6,10 @@ dist: trusty
 git:
   depth: 1
 
+# Using 'cache: cargo' to cache target/ and all of $HOME/.cargo/
+# doesn't work well: the cache is large and it takes several minutes
+# to move it to and from S3. So instead we only cache the mdbook
+# binary.
 cache:
   directories:
     - $HOME/.cargo/bin/