Add "Encrypting Properties" documentation

Update the reference documentation with a section about encrypting
properties and a link to Spring Cloud Vault.

Closes gh-13618

Author: philwebb

spring-boot-project/spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -650,6 +650,21 @@ details.
 
 
 
+[[boot-features-encrypting-properties]]
+=== Encrypting Properties
+Spring Boot does not provide any built in support for encrypting property values, however,
+it does provide the hook points necessary to modify values contained in the Spring
+`Environment`. The `EnvironmentPostProcessor` interface allows you to manipulate the
+`Environment` before the application starts. See <<howto-customize-the-environment-or-application-context>>
+for details.
+
+If you're looking for a secure way to store credentials and passwords, the
+https://cloud.spring.io/spring-cloud-vault/[Spring Cloud Vault] project provides
+support for storing externalized configuration in
+https://www.vaultproject.io/[HashiCorp Vault].
+
+
+
 [[boot-features-external-config-yaml]]
 === Using YAML Instead of Properties
 http://yaml.org[YAML] is a superset of JSON and, as such, is a convenient format for