We will set the CACHE_NAME variable to improve the caching
of various jobs using the same characteristics. Document it
first.

See https://docs.travis-ci.com/user/caching/#caches-and-build-matrices

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 .travis.yml | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/.travis.yml b/.travis.yml
index 640d84f201..c64a551829 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -6,6 +6,12 @@ language: c
 compiler:
   - gcc
 cache:
+  # There is one cache per branch and compiler version.
+  # characteristics of each job are used to identify the cache:
+  # - OS name (currently, linux, osx, or windows)
+  # - OS distribution (for Linux, xenial, trusty, or precise)
+  # - macOS image name (e.g., xcode7.2)
+  # - Names and values of visible environment variables set in .travis.yml or Settings panel
   timeout: 1200
   ccache: true
   pip: true
-- 
2.20.1