Add registered installation instructions (#161)

Author: gdalle

DifferentiationInterface/README.md

@@ -53,7 +53,7 @@ We also provide some experimental backends ourselves:
 
 ## Installation
 
-In a Julia REPL, run
+Until the package is registered, you need to install it from the GitHub repo:
 
 ```julia
 julia> using Pkg
@@ -64,6 +64,14 @@ julia> Pkg.add(
     )
 ```
 
+Once the package is registered, you will be able to do:
+
+```julia
+julia> using Pkg
+
+julia> Pkg.add("DifferentiationInterface")
+```
+
 ## Example
 
 ```jldoctest readme

DifferentiationInterfaceTest/README.md

@@ -29,7 +29,7 @@ Make it easy to know, for a given function:
 
 ## Installation
 
-In a Julia REPL, run
+Until the package is registered, you need to install it from the GitHub repo:
 
 ```julia
 julia> using Pkg
@@ -44,3 +44,11 @@ julia> Pkg.add(
         subdir="DifferentiationInterfaceTest"
     )
 ```
+
+Once the package is registered, you will be able to do:
+
+```julia
+julia> using Pkg
+
+julia> Pkg.add("DifferentiationInterfaceTest")
+```