Includes optional what3words address of the workshop venue (#838)

* Includes optional what3words address of the workshop venue

* Corrects what3words URL in variable description

* Adds description of what3words and why it might be useful.

Moves the what3words variable to sit alongside the other optional
variables, for consistency with the documentation.

_extras/customization.md

@@ -162,6 +162,14 @@ The header may optionally define the following:
     this line.  Note: this value must be given as a string in double
     quotes, rather than as a number.
 
+*   `what3words` is the [what3words](https://what3words.com) address for the
+    workshop venue. What3words divides the world into 3x3m squares and assigns
+    each a unique address consisting of three words separated by dots, allowing
+    users to specify locations precisely. Depending on the venue, this can be
+    used to specify the location of e.g. the room or the building entrance.
+    Note: this value is given in the form 'one.two.three' with no leading
+    slashes and not as a URL.
+
 ### For online workshops
 
 If the workshop is online, follow the same instructions as above with the

index.md

@@ -18,6 +18,7 @@ helper: ["helper one", "helper two"]     # boxed, comma-separated list of helper
 email: ["first@example.org","second@example.org"]    # boxed, comma-separated list of contact email addresses for the host, lead instructor, or whoever else is handling questions, like ["marlyn.wescoff@example.org", "fran.bilas@example.org", "ruth.lichterman@example.org"]
 collaborative_notes:  # optional: URL for the workshop collaborative notes, e.g. an Etherpad or Google Docs document (e.g., https://pad.carpentries.org/2015-01-01-euphoria)
 eventbrite:           # optional: alphanumeric key for Eventbrite registration, e.g., "1234567890AB" (if Eventbrite is being used)
+what3words:           # optional: what3words (https://what3words.com) address of the workshop venue, without leading slashes e.g. "globe.lessening.computers"
 ---
 
 {% comment %} See instructions in the comments below for how to edit specific sections of this workshop template. {% endcomment %}
@@ -174,6 +175,10 @@ address.
   <a href="//www.openstreetmap.org/?mlat={{page.latitude}}&mlon={{page.longitude}}&zoom=16">OpenStreetMap</a>
   or
   <a href="//maps.google.com/maps?q={{page.latitude}},{{page.longitude}}">Google Maps</a>.
+  {% if page.what3words %}
+    What3Words location:
+    <a href="https://what3words.com/{{page.what3words}}">///{{page.what3words}}</a>.
+  {%endif %}
 </p>
 {% elsif online == "true_public" %}
 <p id="where">