docs: update

CONTRIBUTING.md

@@ -41,6 +41,8 @@ request on the repo and our Travis.ci hook will run ShellCheck for you.
 
 ## No no's
 
+- Don’t use `echo`.
+    - Use `printf "%s\n"`
 - Don’t use `bc`.
 - Don’t use `sed`.
     - Use `bash`'s built-in [parameter expansion](http://wiki.bash-hackers.org/syntax/pe).
@@ -52,21 +54,21 @@ request on the repo and our Travis.ci hook will run ShellCheck for you.
 
 ## If Statements
 
-If the test only has one command inside of it; use the compact `if`
+If the test only has one command inside of it; use the compact test
 syntax. Otherwise the normal `if`/`fi` is just fine.
 
 ```sh
 # Bad
 if [[ "$var" ]]; then
-    echo "$var"
+    printf "%s\n" "$var"
 fi
 
 # Good
-[[ "$var" ]] && echo "$var"
+[[ "$var" ]] && printf "%s\n" "$var"
 
 # Also good (Use this for longer lines).
 [[ "$var" ]] && \
-    echo "$var"
+    printf "%s\n" "$var"
 ```
 
 
@@ -77,16 +79,16 @@ Case statements need to be formatted in a specific way.
 ```sh
 # Good example (Notice the indentation).
 case "$var" in
-    1) echo 1 ;;
+    1)  printf "%s\n" 1 ;;
     2)
-        echo 1
-        echo 2
+        printf "%s\n" "1"
+        printf "%s\n" "2"
     ;;
 
     *)
-        echo 1
-        echo 2
-        echo 3
+        printf "%s\n" "1"
+        printf "%s\n" "2"
+        printf "%s\n" "3"
     ;;
 esac
 ```