[docs] Document COMPOSER_HOME

doc/03-cli.md

@@ -199,11 +199,6 @@ directory other than `vendor`.
 By setting this option you can change the `bin` ([Vendor Bins](articles/vendor-bins.md))
 directory to something other than `vendor/bin`.
 
-### COMPOSER_PROCESS_TIMEOUT
-
-This env var controls the time composer waits for commands (such as git
-commands) to finish executing. The default value is 60 seconds.
-
 ### http_proxy or HTTP_PROXY
 
 If you are using composer from behind an HTTP proxy, you can use the standard
@@ -215,4 +210,18 @@ some tools like git or curl will only use the lower-cased `http_proxy` version.
 Alternatively you can also define the git proxy using
 `git config --global http.proxy <proxy url>`.
 
+### COMPOSER_HOME
+
+The `COMPOSER_HOME` var allows you to change the composer home directory. This
+is a global (per-user on the machine) hidden directory that caches repository
+data.
+
+By default it points to `$HOME/.composer` on *nix and `$APPDATA/Composer` on
+Windows.
+
+### COMPOSER_PROCESS_TIMEOUT
+
+This env var controls the time composer waits for commands (such as git
+commands) to finish executing. The default value is 60 seconds.
+
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