update the nu-themes readme (nushell#652)

related to
-
https://discord.com/channels/601130461678272522/601130461678272524/1167879553918705705

## description
this PR updates the README of the `nu-themes` module in the `nu-scripts`
package:
- fix the path with `themes/nu-themes/` instead of `themes/themes/`
- adds some instructions to use `nupm`

> **Note**
> see the [new README
here](https://github.com/amtoine/nu_scripts/tree/update-nu-themes-readme/themes#use-a-colorscheme)

themes/README.md

@@ -3,20 +3,45 @@
 Credit to @lemnos and [all contributors](https://github.com/lemnos/theme.sh/blob/master/CREDITS.md).
 
 ## Use a colorscheme
+### The manual way
 Currently, the theme change has to be a bit manual.
 To have access to the list of all the themes,
 ```rust
-ls ./themes/themes
+ls ./themes/nu-themes
 ```
 
 To use the `dracula` theme for instance, please run
 ```rust
-use ./themes/themes/dracula.nu
+use ./themes/nu-themes/dracula.nu
 $env.config = ($env.config | merge {color_config: (dracula)})
 ```
 
 The theme should be activated!
 
+### Using Nupm
+- install [Nupm] by following the [Nupm instructions]
+- download the `nu_scripts` repository
+```shell
+git clone https://github.com/nushell/nu_scripts
+```
+- activate the `nupm` module with `use nupm`
+- install the `nu-scripts` package
+```nushell
+nupm install --path --force nu_scripts
+```
+
+> **Note**
+> installing the `nu-scripts` package will install `nu-themes` and other modules
+
+- now, add the following in your config
+```nushell
+use nu-themes/nushell-dark.nu
+$env.config.color_config = (nushell-dark)
+```
+
 ## Screenshots
 
 Here are [screenshots](./screenshots/README.md) of our themes.
+
+[Nupm]: https://github.com/nushell/nupm
+[Nupm instructions]: https://github.com/nushell/nupm#recycle-installation-toc