Added a short guide to help creating the Symfony diagrams

_images/sources/README.md

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+How to Create Symfony Diagrams
+==============================
+
+Creating the Diagram
+--------------------
+
+* Use [Dia][1] as the diagramming application;
+* Use [PT Sans Narrow][2] as the only font in all diagrams (if possible, use
+  only the "normal" weight for all contents);
+* Use 36pt as the base font size;
+* Use 0.10 cm width for lines and shape borders;
+* Use the following color palette:
+  * Text, lines and shape borders: black (#000000)
+  * Shape backgrounds:
+    * Grays: dark (#4d4d4d), medium (#b3b3b3), light (#f2f2f2)
+    * Blue: #b2d4eb
+    * Red: #ecbec0
+    * Green: #b2dec7
+    * Orange: #fddfbb
+
+In case of doubt, check the existing diagrams or ask to the
+[Symfony Documentation Team][3].
+
+Saving and Exporting the Diagram
+--------------------------------
+
+* Save the original diagram in `*.dia` format in `_images/sources/<folder-name>`;
+* Export the diagram to SVG format and save it in `_images/<folder-name>`.
+
+Including the Diagram in the Symfony Docs
+-----------------------------------------
+
+Use the following snippet to embed the diagram in the docs:
+
+```
+.. raw:: html
+
+    <object data="../_images/<folder-name>/<diagram-file-name>.svg" type="image/svg+xml"></object>
+```
+
+Reasoning
+---------
+
+* Dia was chosen because it's one of the few applications which are free, open
+  source and compatible with Linux, macOS and Windows.
+* Font, colors and line widths were chosen to be similar to the diagrams used
+  in the best tech books.
+
+Troubleshooting
+---------------
+
+* On some macOS systems, Dia cannot be executed as a regular application and
+  you must run the following console command instead:
+  `export DISPLAY=:0 && /Applications/Dia.app/Contents/Resources/bin/dia`
+
+[1]: http://dia-installer.de/
+[2]: https://fonts.google.com/specimen/PT+Sans+Narrow
+[3]: https://symfony.com/doc/current/contributing/code/core_team.html