docs: describe using map around objects in the gotchas section

Author: josdejong

README.md

@@ -613,6 +613,14 @@ $ cat users.json | jsonquery 'sort(.age)' > output.json
 
 The JSON Query language has some gotchas. What can be confusing at first is to understand how data is piped through the query. A traditional function call is for example `max(myValues)`, so you may expect to have to write this in JSON Query like `["max", "myValues"]`. However, JSON Query has a functional approach where we create a pipeline like: `data -> max -> result`. So, you will have to write a pipe which first gets this property and next calls the function max: `.myValues | max()`.
 
+Another gotcha is that unlike some other query languages, you need to use the `map` function to pick some properties out of an array _for every item_ in the array. When you're just picking a few fields without renaming them, you can use the function `pick` too, which is more concise.
+
+```
+.friends | { firstName: .name, age: .age }        WRONG 
+.friends | map({ firstName: .name, age: .age })   RIGHT 
+.friends | pick(.name, .age)                      RIGHT 
+```
+
 ## Development
 
 To develop, check out the repo, install dependencies once, and then use the following scripts: