zig fetch: add --save flag

```
--save        Add the fetched package to build.zig.zon
--save=[name] Add the fetched package to build.zig.zon as name
```

lib/init/build.zig.zon

@@ -15,8 +15,7 @@
     // Once all dependencies are fetched, `zig build` no longer requires
     // Internet connectivity.
     .dependencies = .{
-        // A future version of Zig will provide a `zig add <url>` subcommand
-        // for easily adding dependencies.
+        // See `zig fetch --save <url>` for a command-line interface for adding dependencies.
         //.example = .{
         //    // When updating this field to a new URL, be sure to delete the corresponding
         //    // `hash`, otherwise you are communicating that you expect to find the old hash at
@@ -36,7 +35,7 @@
         //
         //    // When this is provided, the package is found in a directory relative to the
         //    // build root. In this case the package's hash is irrelevant and therefore not
-        //    // computed.
+        //    // computed. This field and `url` are mutually exclusive.
         //    .path = "foo",
         //},
     },
@@ -57,5 +56,7 @@
         //"build.zig",
         //"build.zig.zon",
         //"src",
+        //"LICENSE",
+        //"README.md",
     },
 }