docs: document --package-json flag (2.8)

runtime/reference/cli/add.md

@@ -42,3 +42,16 @@ deno add --npm express
 If your project has a `package.json`, npm packages will be added to
 `dependencies` in `package.json`. Otherwise, all packages are added to the
 `imports` field in [`deno.json`](/runtime/fundamentals/configuration/).
+
+To force every dependency to be written to `package.json` (creating one if
+needed), pass `--package-json` (Deno 2.8+):
+
+```sh
+deno add --package-json npm:express jsr:@std/path
+```
+
+JSR packages added with `--package-json` are written in their npm-compatible
+form (`npm:@jsr/...`). The same flag works on
+[`deno install`](/runtime/reference/cli/install/),
+[`deno remove`](/runtime/reference/cli/remove/), and
+[`deno uninstall`](/runtime/reference/cli/uninstall/).

runtime/reference/cli/install.md

@@ -46,6 +46,22 @@ If your project has a `package.json` file, the packages coming from npm will be
 added to `dependencies` in `package.json`. Otherwise all packages will be added
 to `deno.json`.
 
+### deno install --package-json
+
+By default, Deno picks the configuration file to write to (`deno.json` or
+`package.json`) based on which one is closest to the current working directory.
+Starting in Deno 2.8, `--package-json` forces dependencies to be written to
+`package.json`, regardless of any nearby `deno.json`. If no `package.json`
+exists yet, one is created.
+
+```sh
+deno install --package-json npm:express jsr:@std/path
+```
+
+JSR packages added with `--package-json` are written in their npm-compatible
+form (`npm:@jsr/...`). The same flag works on `deno add`, `deno remove`, and
+`deno uninstall`.
+
 ### deno install --entrypoint [FILES]
 
 Use this command to install all dependencies that are used in the provided files