chore: Add documentation on environment attestor (#555)

Signed-off-by: Matthias Glastra <[email protected]>

docs/attestors/environment.md

@@ -1,8 +1,35 @@
 # Environment Attestor
 
+> Warning! Environment variables can contain SENSITIVE data that should not be
+> shown. Review variables in your environment and use the `--env-add-sensitive-key`
+> to exclude them from the capture if they are not covered by the default
+> sensitive vars.
+
 The Environment Attestor records the OS, hostname, username, and all environment variables set
-by TestifySec Witness at execution time.  Currently there is no means to block specific environment variables
-so take care to not leak secrets stored in environment variables.
+at execution time. The default mode is running in obfuscation mode that will capture all variables
+but it will obfuscate any variable that is in the sensitive vars list.
+
+## Filter instead of obfuscate
+
+When you use `--env-filter-sensitive-vars` it will remove sensitive vars completely
+from the list.
+
+## Adding additional sensitive keys
+
+If you want to add keys to the sensitive list, either specific or with a glob,
+you can use `--env-add-sensitive-key 'FOO'` for `FOO` or
+ `--env-add-sensitive-key 'FOO*'` to also capture a variable like `FOO_BAR`.
+
+## Explicitly allow sensitive key
+
+There could be cases where you really want to have a specific key that is part
+of the default sensitive vars list to be captured. You can do so by using the
+`--env-exclude-sensitive-key`.
+
+## Default sensitive vars
+
+The attestor has a default sensitive vars list. You can find the list in the
+code base [here](https://github.com/in-toto/go-witness/blob/main/environment/sensitive_env_vars.go).
 
 ## Schema
 ```json