add links to more resources (#645)

Author: baggepinnen

docs/src/examples/example.md

@@ -222,3 +222,8 @@ save_docs_plot("pidplotsgof2.svg"); # hide
 ```
 ![](../../plots/pidplotsnyquist2.svg)
 ![](../../plots/pidplotsgof2.svg)
+
+# Further examples
+See the [examples folder](https://github.com/JuliaControl/ControlSystems.jl/tree/master/example) as well as the notebooks in [ControlExamples.jl](https://github.com/JuliaControl/ControlExamples.jl).
+
+See also [the paper introducing the toolbox](https://portal.research.lu.se/en/publications/controlsystemsjl-a-control-toolbox-in-julia) with [supplementary material](https://github.com/JuliaControl/CDC2021).

docs/src/index.md

@@ -32,3 +32,9 @@ Depth = 2
 Pages = ["lib/constructors.md",  "lib/analysis.md", "lib/syntheis.md", "lib/timefreqresponse.md", "lib/plotting.md"]
 Depth = 1
 ```
+
+## Ecosystem
+The JuliaControl and surrounding ecosystem contains a few additional packages that may be of interest
+- [RobustAndOptimalControl.jl](https://github.com/JuliaControl/RobustAndOptimalControl.jl)
+- [SymbolicControlSystems.jl](https://github.com/JuliaControl/SymbolicControlSystems.jl)
+- [ControlSystemIdentification.jl](https://github.com/baggepinnen/ControlSystemIdentification.jl)

docs/src/lib/constructors.md

@@ -2,7 +2,7 @@
 Pages = ["constructors.md"]
 ```
 
-# Constructing transfer functions
+# Constructing systems
 
 ```@docs
 append
@@ -16,4 +16,5 @@ sminreal
 ss
 tf
 zpk
+delay
 ```