add some basic docs for weak deps

docs/make.jl

@@ -35,6 +35,7 @@ makedocs(
         "managing-packages.md",
         "environments.md",
         "creating-packages.md",
+        "weakdeps.md",
         "compatibility.md",
         "registries.md",
         "artifacts.md",

docs/src/weakdeps.md

@@ -0,0 +1,77 @@
+# Weak dependencies
+
+It is sometimes desirable to be able to extend some functionality of a package without having to
+unconditionally take on the cost (in terms of e.g. load time) of adding an extra dependency.
+A *weak* dependency is a package that is only available to load if some other package in the
+current environment has that package as a normal (or strong) dependency.
+
+Weak dependencies are listed in a `Project.toml` file under the `[weakdeps]` section which can be compared to a
+(strong) dependency which is under the `[deps]` section.
+Compatibility on weak dependencies is specified like a normal dependency in the `[compat]` section.
+
+A useful application of weak dependencies could be for a plotting package that should be able to plot
+objects from different Julia packages. Adding all those different Julia packages as dependencies
+could be expensive. Instead, these packages are added as weak dependencies so that they are available only
+if there is a chance that someone might call the function with such a type.
+
+Below is an example of how the code can be structured for a use case as outlined above.
+
+`Project.toml`:
+```toml
+name = "Plotting"
+version = "0.1.0"
+uuid = "..."
+
+[deps] # strong dependencies
+Colors = "5ae59095-9a9b-59fe-a467-6f913c188581"
+
+[weakdeps]
+Contour = "d38c429a-6771-53c6-b99e-75d170b6e991"
+
+[compat] # compat can also be given on weak dependencies
+Colors = "0.12.8"
+Contour = "0.6.2"
+```
+
+`src/Plotting.jl`:
+```julia
+module Plotting
+
+using Colors
+
+if Base.@hasdep Contour
+    using Contour
+    function plot(c::Contour.ContourCollection
+         ...
+    end
+end
+
+end # module
+```
+
+## Compatibility with older Julia versions.
+
+It is possible to have a dependency be a weak version in Julia versions that support it and be a strong dependency in earlier
+Julia versions. This is done by having the dependency as *both* a strong and weak dependency. Older Julia versions will ignore
+the specification of the dependency as weak while new Julia versions will tag it as a weak dependency.
+
+The above code would then look like this:
+
+```julia
+module Plotting
+
+using Colors
+
+if !isdefined(Base, :hasdep) || Base.hasdep(@__MODULE__, :Contour)
+    using Contour
+    function plot(c::Contour.ContourCollection
+         ...
+    end
+end
+
+end # module
+```
+
+where the "conditional code" is executed unconditionally on old Julia versions and based on the presence of the weak
+dependency on the new Julia version. Here the functional form of `@hasdep` was used which requires a module as the first
+argument.