add documentation about Travis token (#1902)

.travis.yml

@@ -13,5 +13,7 @@ after_failure:
 
 env:
   global:
-  - secure: ewQMcFd3wH8IHOeGhIkBr2kAgadONmesv2KB+9bPRCZ5gvtFFwlkCMdPnQBR9Kd3GKC8r+EO0JNNFWOvrDjPYhEHi9Ab26Q/qodwfwne9YloKXT+C48Zrbrj8q7kb/FeMYPev+RMuM96j5E5QyqBVohGP7hC2bpU3mvAhQ2wBCE=
+  # Used to upload the tldr archive to tldr-pages repo.
+  # Achieved via the upload_assets() function of scripts/build.sh
+  # This is an encrypted form of @agnivade's user token.
   - secure: AJPra/q3bCFHzMOam1aFz4tzasYuU261Mk6lISh1VJatibHa7nBErsuA3VbR5qth9LblH5HFmNGl4bwmas/PTD1P3lPAHO19gdlMb1kpS9MhTojQP/0EPCsyMTgnWcmNMU2XMvYGHFT0JFn4vj/0TrM9CUMDoT9WhtnVJfgRrlY=

scripts/build.sh

@@ -1,5 +1,6 @@
 #!/usr/bin/env bash
 
+# This script is executed by Travis CI when a PR is merged (i.e. in the `after_success` step).
 set -ev
 
 function initialize {
@@ -28,6 +29,7 @@ function build_archive {
 }
 
 function upload_assets {
+  # ${GH_TOKEN} is defined as a secure variable inside .travis.yml
   git clone --quiet --depth 1 https://${GH_TOKEN}@${SITE_URL} $SITE_HOME
   mv -f $TLDR_ARCHIVE $SITE_HOME/assets/
   cp -f $TLDRHOME/pages/index.json $SITE_HOME/assets/