Added extension points

Needs "Contribution" page at the beginning of the book

chapters/004_GameDev_Basics/005_Game_Design_Tips/004_Rewarding_Players.md

@@ -97,8 +97,6 @@ No matter how the 1-UP is achieved, this should be celebrated with a jingle that
 
 Some games even go as far as temporarily pause the game while the (short) jingle plays, that how important an extra-life is: "Stop everything! We got a 1-UP here!".
 
-{{placeholder}}
-
 #### Other approaches
 
 There are different approaches to a "lives system" that don't necessarily involve lives. The main objective is creating a mechanic that rewards the player for doing the right thing and punish them for doing the wrong thing (although not too harshly, or the player will stop playing).
@@ -107,4 +105,6 @@ One such approach was used in the first System Shock (1994) game: the space stat
 
 Here's the catch: the "cyborg conversion chambers" are fashioned out of "restoration bays", that means that disabling the cyborg conversion process will allow the player to be immediately resurrected at the nearest restoration bay, although they won't start at full health. This rewards the player for finding such restoration bays by giving them essentially infinite lives.
 
+{{extend}}
+
 <!-- TODO: Talk about other level design tips -->

chapters/004_GameDev_Basics/005_Game_Design_Tips/008_Tips.md

@@ -134,9 +134,7 @@ In the game "The Witcher III", precisely in patch 1.05, a mechanic to prevent su
 On the flip side, higher level players could exploit this endlessly respawning monster to gain Chort Hides, which are worth more currency than Cow Hides. This was patched by making only one Chort spawn.
 ::::::::::::::::
 
-{{placeholder}}
-
-<!-- TODO: Finish talking about farming -->
+{{extend}}
 
 #### Leveling Curves
 

chapters/005_Assets/001_Making_Assets/003_Sounds_Music/009_Tips.md

@@ -8,6 +8,6 @@ Sometimes we may find our audio samples lacking that "punch" they would need, th
 
 It's better to cut the higher frequencies instead, and eventually boost the entire volume of the sample during mixing. This way the nature of the sample doesn't get tainted by boosting, and we obtain the result we wanted.
 
-{{placeholder}}
+{{extend}}
 
 <!-- TODO: Give some tips to process audio files -->

chapters/005_Assets/001_Making_Assets/003_Sounds_Music/012_Rhythms.md

@@ -30,6 +30,6 @@ By using hi-hats, a kick drum and a snare drum, you can create a very simple roc
 
 ![A simple rock beat](./images/resources/simple_rock_beat.png){width=50%}
 
-{{placeholder}}
+{{extend}}
 
 <!-- TODO: Teach more basic rhythms to get people going -->

chapters/007_Refining/002_Accessibility.md

@@ -114,6 +114,6 @@ Some special cases
 
 A special mention goes to the VR game "Moss", where a character (named Quill) communicates both emotionally and gives clues on the puzzles using the American Sign Language (ASL).
 
-{{placeholder}}
+{{extend}}
 
 <!-- TODO: More talk about accessibility -->

chapters/007_Refining/004_Optimization_Profiling/002_Profiling.md

@@ -69,6 +69,6 @@ If you suspect a memory leak, you may want to take a look at these sections:
 - [Using memory analyzers to detect leaks](#mem_analyzers)
 - [Resource Pools](#res_pools)
 
-{{placeholder}}
+{{extend}}
 
 <!-- TODO: Continue with more investigation suggestions -->

chapters/007_Refining/004_Optimization_Profiling/003_Optimizing.md

@@ -217,4 +217,4 @@ It is a lot harder for a database to deal with multiple small queries than deali
 
 This doesn't apply only to databases, but also other data structures that allow for filtering.
 
-{{placeholder}}
+{{extend}}

filters/placeholder.lua

@@ -2,6 +2,8 @@ local List = require 'pandoc.List'
 function placeholder(elem)
     if elem.text == "{{placeholder}}" then
         return pandoc.Strong {pandoc.Emph {pandoc.Str "[This section is a work in progress and it will be completed as soon as possible]"}}
+    elseif elem.text == "{{extend}}" then
+        return pandoc.Strong {pandoc.Emph {pandoc.Str "[Do you know more about this matter? You can contribute to this book!]"}}
     else
         return elem
     end