Add note on setting environment variables in `cmd.exe` to wiki

[mataha]

Directly related to #567. Feel free to use the following patch as you deem fit:

diff --git a/HOWTO:-set-environment-variables.md b/HOWTO:-set-environment-variables.md
index 56b6b3e2c0de..1b85e955d3b0 100644
--- a/HOWTO:-set-environment-variables.md
+++ b/HOWTO:-set-environment-variables.md
@@ -7,6 +7,13 @@ export _ZO_ECHO='1'
 ```
 </details>
 <details>
+<summary>`cmd.exe`</summary>
+Add this to your configuration or AutoRun command:
+```batchfile
+set _ZO_ECHO=1
+```
+</details>
+<details>
 <summary>Elvish</summary>
 Add this to your configuration (usually `~/.elvish/rc.elv`):
 ```sh

Note: the above sets a variable à la export; permanently writing it to an environment (much like /etc/environment in Linux) would imply using setx like so:

setx _ZO_ECHO 1

[ajeetdsouza]

Added, thanks! I'll try to work on the cmd PR as soon as I get time.