docs/env_variables: document PMB_FDE_PASSWORD

Co-authored-by: Oliver Smith <ollieparanoid@postmarketos.org>
Part-of: !2538

docs/environment_variables.md

@@ -13,3 +13,11 @@ APKINDEX results in a 404 not found error. This is used by
 [bpo](https://build.postmarketos.org) when building a new stable repository for
 the first time. For example if the `x86_64` repository was already built and
 published, but the `aarch64` repository wasn't published yet.
+
+## `PMB_FDE_PASSWORD`
+
+This variable can be used to set the password when running `install --fde`. The
+password is written to a temporary file and can be read from
+`/proc/<pid>/environ`. Make sure you are aware of the security implications,
+consider using this feature only with test passwords or in environments such as
+live operating systems running in memory.