Update Travis notes

doc/travis_notes.txt

@@ -5,14 +5,15 @@ Travis Notes:
 - Use http://lint.travis-ci.org/ to make sure it is valid yaml.
 - Make sure all of your "-" lines start on the same column
 - Make sure all of your "if" lines are aligned with the "else" and "fi" lines
+- Recommend using tab stops (but not tab chars) everywhere for alignment
 - "If" blocks must be on one travis statement and have semicolons at the end of each line:
 
 ```
-    - if [[ $TRAVIS_PYTHON_VERSION == 2.7 ]]; then
-        echo "2.7";
-      else
-        echo "Not 2.7";
-      end
+    -   if [[ $TRAVIS_PYTHON_VERSION == 2.7 ]]; then
+            echo "2.7";
+        else
+            echo "Not 2.7";
+        end
 ```
 
 - "If" blocks cannot contain comments
@@ -21,5 +22,8 @@ Travis Notes:
 `echo "hello : world"` is interpreted as `echo \"hello : world\"`
 `"echo 'hello : world'"` is interpreted as `echo 'hello : world'`
 `"echo 'hello : '$(MYVAR)'world'"` is interpreted as `echo 'hello : $(MYVAR)world'`
-- Use `travis_retry` before a command to have it try several times before failing
-  (useful for installing from third party sources)
+- Use `travis_retry` before a command to have it try several times before
+failing (useful for installing from third party sources)
+- Feel free to cancel a build rather than waiting for it to go to completion
+if you have made a change to that branch.
+- A VM with 64bit Ubuntu 12.04 is a huge help.