Start unused parameters user documentation

Author: nicolasstucki

docs/docs/reference/unused-terms.md

@@ -0,0 +1,58 @@
+---
+layout: doc-page
+title: "Unused Parameters"
+---
+
+Why unused parameters?
+----------------------
+TODO
+
+
+What are unused parameter?
+--------------------------
+Unused parameter are values that are know not to be used. Parameters of methods and functions 
+can be declared as unused. Those parameters wont be usable for computations, thought they can 
+be used as arguments to other unused parameters.
+
+```scala
+def methodWithUnusedEv(unused ev: Ev): Int = 42
+
+val lambdaWithUnusedEv: unused Ev => Int = 
+  unused (ev: Ev) => 42
+```
+
+Not only parameters can be marked as unused, `val` and `def` can also be marked with `unused`.
+The will also only be usable as arguments to `unused` parameters.
+
+```scala
+unused val unusedEvidence: Ev = ...
+methodWithUnusedEv(unusedEvidence)
+```
+
+
+What happens with unused values at runtime?
+-------------------------------------------
+As `unused` are guaranteed not to be used in computations, they can and will be erased.
+
+```scala
+// becomes def methodWithUnusedEv(): Int at runtime
+def methodWithUnusedEv(unused ev: Ev): Int = ...  
+
+def evidence1: Ev = ...
+unused def unusedEvidence2: Ev = ... // does not exist at runtime
+unused val unusedEvidence3: Ev = ... // does not exist at runtime
+
+// evidence1 is evaluated but the result is not passed to methodWithUnusedEv
+methodWithUnusedEv(evidence1)
+
+// unusedEvidence2 is not evaluated and its result is not passed to methodWithUnusedEv
+methodWithUnusedEv(unusedEvidence2)
+```
+
+Examples
+--------
+TODO
+
+```scala
+
+```