warn about the staff situation

tsa/howto/conference.mdwn

@@ -137,6 +137,12 @@ Some observations:
    reliable BBB will be in production. preliminary reports of BBB
    admins seem to say it's fairly stable and doesn't require much work
    after the complex install procedure
+ * current TPA situation is that we will be understaffed by 50%
+   starting on May 1st 2020, and by 75% for two months or more during
+   the summary. it will not be possible to complete such a project
+   with monthly, recurring hours if that situation is not fixed, and
+   would have been difficult to complete the project even before the
+   staff changes.
 
 ### Mumble