Add explanation of Git configuration step

.appveyor.yml

@@ -20,6 +20,7 @@ build_script:
   - echo "%JL_BUILD_SCRIPT%"
   - C:\julia\bin\julia -e "%JL_BUILD_SCRIPT%"
 test_script:
+  # Git configuration is required to make commits in generated packages.
   - git config --global user.name "AppVeyor"
   - git config --global user.email "appveyor@example.com"
   - echo "%JL_TEST_SCRIPT%"

.travis.yml

@@ -13,6 +13,7 @@ matrix:
 notifications:
   email: false
 before_script:
+  # Git configuration is required to make commits in generated packages.
   - git config --global user.name "Travis"
   - git config --global user.email "travis@example.com"
 after_success: