Add instructions to PlantUML README to set exec-mode accordingly

CHANGELOG.develop

@@ -2305,6 +2305,8 @@ Other:
     the =php= layer's =php-backend= variable to =lsp=
 **** PlantUML
 - Added a missing prefix: =compile=
+- Added instructions for compiling the image locally by setting
+  =plantuml-default-exec-mode= to =jar= (thanks to Daniel Caixinha)
 **** Platinum
 - Added the =.plum= extension to the platinum modes (thanks to Sylvain Benner)
 - Remove automatic indentation on paste (thanks to Sylvain Benner)

layers/+lang/plantuml/README.org

@@ -58,6 +58,12 @@ and configure the =plantuml-jar-path= respectively:
 To get the full range of =Plantuml= compilations working, you will also need
 the native package [[http://graphviz.org/][graphviz]] installed on your system.
 
+Additionally, to compile the image locally, you need to set =plantuml-default-exec-mode= to =jar=, e.g.:
+
+#+BEGIN_SRC emacs-lisp
+  (setq plantuml-default-exec-mode 'jar)
+#+END_SRC
+
 * Org-Babel Integration
 To enable the execution of embedded plantuml code blocks within [[http://orgmode.org/][Org-Mode]]
 documents, define a value for =org-plantuml-jar-path= in your =~/.spacemacs=: