Making one is easy, here is the file structure (Yes it's a table):

pikachu.callouts.maplist = {
	{name = "CT Spawn", pos = {{14,29},{25,36}}},
	-- Add new entries here
}

Each entry has the following format:

{name = n, pos = {{x1,y1},{x2,y2}}},

n is the callout name (Yes you can have multiple entries with the same name)
x1 and y1 are the x and y tile positions of the top left region that will be designated as n
x2 and y2 are the x and y tile positions of the bottom right region that will be designated as n

Since it is a script, it is possible to add other pieces of code after the table, whatever it may be.
If saving as a map Lua script (in *CS2D folder*\maps), add

loadedfromfile = true

after the table! Othewise it will cause problems!

1. Place a info_teamgate or info_nobuildings entity on the top left region to be assigned the callout
2. Click on the bottom right region to finish setting up the callout zone
3. Do either one of these (Script prioritises the 1st, which I recommend):
Write the callout name under the Name text box following this format: pika_name (where name is the callout name).
Write the callout name under the Trigger text box

3.7
Included map callout data file is for de_dust2_classic
3.6
Changed priority order for generating callout names from entities
3.5
Patched advanced radio,teamchat script (21) does not work if executed before this script, or by itself
3.4
Map callout location does not appear under the radar immediately after spawning until you move
3.3
Writing the name under the Trigger text box for the Info_TeamGate entity has no effect on the script
3.2
Patched advanced radio,teamchat script (21) for full compatibility with this script
3.1
Callout entities will now only be disabled by the script if not disabled prior (Encountered bug while modifying advanced radio,teamchat script (21))
3.0
Thanks GeoB99 for the following suggestions!
Reads from Info_NoBuildings as well in the similar method as Info_TeamGate
Compatibility with advanced radio,teamchat script (21)
Re-worked code using advanced radio,teamchat script (21) as a basis
2.0
Rainoth 's suggestion on reading callout data from entities
1.5
Limited support for map data files in the *CS2D folder*\maps folder