🐛 Fix rustup use completions and modify README (#696)

Hey, it's me, I finally fixed this `use` vs `source` script.nu 😄 

Also, I added a few lines of the `custom-completions/README.md` to
explain what happened in my case

custom-completions/README.md

@@ -4,6 +4,20 @@ This current directory provides custom completions. They can be used by importin
 
 ```nushell
 use path/to/<command>/<command>-completions.nu *
+# or
+source path/to/<command>/<command>-completions.nu
+# without the `*` at the end
 ```
 
 With `path/to/<command>` being either the relative path of the file to your current working directory or its absolute path.
+
+Bear in mind that if you import the `use <path> *`, it is important that you don't name a function with the same name of the file
+
+```nu
+# file: rustup.nu 
+
+export extern rustup [
+    ...args
+]
+```
+`use ./rustup.nu *` won't work here

custom-completions/rustup/rustup-completions.nu

@@ -131,7 +131,7 @@ def "nu-complete rustup completions shell" [] {
 # ------------------ export extern commands ------------------
 
 export extern "rustup" [
-   command?: string@"nu-complete rustup"
+  command?: string@"nu-complete rustup"
     --verbose(-v)    # Enable verbose output
     --quiet(-q)      # Disable progress output
     --help(-h)       # Print help information