Add 'Tab completion' to the Glossary (#918)

Author: maxim-belkin

_episodes/02-numpy.md

@@ -377,7 +377,8 @@ standard deviation: 4.61383319712
 >
 > How did we know what functions NumPy has and how to use them?
 > If you are working in IPython or in a Jupyter Notebook, there is an easy way to find out.
-> If you type the name of something followed by a dot, then you can use tab completion
+> If you type the name of something followed by a dot, then you can use
+> [tab completion]({{ page.root }}/reference.html#tab-completion)
 > (e.g. type `numpy.` and then press <kbd>Tab</kbd>)
 > to see a list of all functions and attributes that you can use. After selecting one, you
 > can also add a question mark (e.g. `numpy.cumprod?`), and IPython will return an

reference.md

@@ -254,6 +254,10 @@ syntax error
 :   A programming error that occurs when statements are in an order or contain characters
     not expected by the programming language.
 
+tab completion
+:   A feature of command-line interpreters, in which the program automatically fills in partially
+    typed commands upon pressing the <kbd>Tab</kbd> key.
+
 test oracle
 :   A program, device, data set, or human being
     against which the results of a test can be compared.