Convert README for manual images to Markdown

I also updated the filenames for the images, changed the recommended
format for screenshots other than "fullscreen" screenshots to PNG,
and removed the (no longer true) claim that the example places to create
the screenshots match where the English screenshots were taken.

Resolves #2856.

[ci skip]

doc/manual/images/README.md

@@ -0,0 +1,53 @@
+This document lists some general guideline on taking the screenshots for the
+manual. Please adopt it to the correct version of the time you are taking the
+screenshot and make sure to point the entry in the .po file to the correct
+screenshot.
+
+The examples we give are based on the availability of the [ImageMagick](https://www.imagemagick.org/script/index.php) tools
+used for converting the images to the JPEG format.
+
+
+Here is a list of the general howto for the screenshots:
+
+1) Take the "original" version as PNG or BMP.
+
+2) Take the fullscreen screenshots in 1024x768.
+
+3) Afterwards, convert the fullscreen screenshots to JPEG and change the
+resolution to 640x480 and set quality level to 80%. This is the command to use
+when using ImageMagick (change file name accordingly for the titlescreen
+screenshot):
+```sh
+convert -resize 640x480 -quality 80 game-screen-1.13.11+dev.bmp game-screen-1.13.11+dev.jpg
+```
+
+4) For those "parts of the screen" screenshots just copy out the relevant part
+from the full versions. All "non fullscreen" screenshots should be stored as
+PNG and in the original size. The ImageMagick command would look like this:
+```sh
+convert multiplayer-1.13.11+dev.bmp multiplayer-1.13.11+dev.png
+```
+
+5) Send the files in once you translated the strings in the po file. In the
+translation use your locale as prefix/foldername. So if you were eg translating
+to German, you would use 'de' as foldername where the images are in and the
+filename for the game-screen screenshot would be this:
+    * de/game-screen-1.13.11+dev.jpg
+    * de/multiplayer-1.13.11+dev.png
+
+    Please send the screenshots in a separate archive.
+
+
+As a hint where you could create the screenshots:
+* `game-screen-1.13.11+dev.jpg`: 3rd scenario of AToTB, just jump there with
+  :debug and :n and then let the AI play some turns with :droid
+* `right_pane-1.13.11+dev.png`: right part from the "game-screen" screenshot
+* `top_pane-1.13.11+dev.png`: top part from the "game-screen" screenshot
+* `main-menu-1.13.11+dev.jpg`: the main menu (obvious, isn't it?)
+* `multiplayer-1.13.11+dev.png`: just the dialog that pops up when clicking on
+  'multiplayer' in the main menu, take care to set the player name to the
+  default value 'player' (or whatever it is in your lang, since this one is a
+  translatable string)
+* `recruit-1.13.11+dev.png`: start a skirmish game and select default era and
+  rebels as your race, click on recruit and take a screenshot of the dialog
+  popping up