feat(transfer): allow encryption of uploads using GPG (#9983)

Co-authored-by: Marc Cornellà <hello@mcornella.com>

plugins/transfer/README.md

@@ -2,23 +2,40 @@
 
 [`transfer.sh`](https://transfer.sh) is an easy to use file sharing service from the command line
 
-## Usage
+To use it, add `transfer` to the plugins array in your zshrc file:
 
-Add `transfer` to your plugins array in your zshrc file:
 ```zsh
 plugins=(... transfer)
 ```
 
-Then you can:
+## Usage
 
-- transfer a file:
+- Transfer a file: `transfer file.txt`.
 
-```zsh
-transfer file.txt
-```
+- Transfer a whole directory (it will be automatically compressed): `transfer dir`.
 
-- transfer a whole directory (it will be automatically compressed):
+### Encryption / Decryption
 
-```zsh
-transfer directory/
-```
+- Encrypt and upload a file with symmetric cipher and create ASCII armored output:
+
+  ```zsh
+  transfer file -ca
+  ```
+
+- Encrypt and upload directory with symmetric cipher and gpg output:
+
+  ```zsh
+  transfer directory -ca
+  ```
+
+- Decrypt file:
+
+  ```zsh
+  gpg -d file -ca
+  ```
+
+- Decrypt directory:
+
+  ```zsh
+  gpg -d your_archive.tgz.gpg | tar xz
+  ```