Update usage section of README for Julia 1.x (#673)

* Update `Usage` section of README to Julia 1.x

The README was still talking about a line in the `REQUIRE` file. Replace 
that with the respective lines for `Project.toml`. Also remove the 
obsolve `import Compat.String` usage example.

* Replace dead link in README

The latest Julia manual has no detailed discussion of packages; it has 
moved to the Pkg.jl docs.

README.md

@@ -13,18 +13,21 @@ Compat package provides a macro that lets you use the *latest syntax*
 in a backwards-compatible way.
 
 This is primarily intended for use by other [Julia
-packages](http://docs.julialang.org/en/latest/manual/packages/), where
+packages](https://julialang.github.io/Pkg.jl/v1/creating-packages/), where
 it is important to maintain cross-version compatibility.
 
 ## Usage
 
-To use Compat in your Julia package, add a line `Compat` to the
-`REQUIRE` file in your package directory.  Then, in your package,
-shortly after the `module` statement include lines like these:
+To use Compat in your Julia package, add a line
+`Compat = "34da2185-b29b-5c13-b0c7-acf172513d20"` in the `[deps]` section
+and a line `Compat = "..."` in the `[compat]`section to the `Project.toml` file
+in your package directory. The version in the latter should be the minimum
+version that supports all needed fatures (see list below), and (if applicable)
+any newer major versions verified to be compatible. Then, in your package,
+shortly after the `module` statement a line like this:
 
 ```julia
 using Compat
-import Compat.String
 ```
 
 and then as needed add