Add postbuild documentation

_docs/Build_Metadata_Reference.md

@@ -100,6 +100,7 @@ The following sections describe the fields recognised within the file.
 * [_Builds.gradleprops_](#build_gradleprops)
 * [_Builds.antcommands_](#build_antcommands)
 * [_Builds.output_](#build_output)
+* [_Builds.postbuild_](#build_postbuild)
 * [_Builds.novcheck_](#build_novcheck)
 * [_Builds.antifeatures_](#build_antifeatures)
 * [_AllowedAPKSigningKeys_](#AllowedAPKSigningKeys)
@@ -773,6 +774,23 @@ configuration to the build. These are (roughly in order of application):
 
  This runs in [`subdir:`](#build_subdir) if set.
 
+`postbuild: xxxx`<a name="build_postbuild"></a>
+
+: As for 'prebuild', but runs after the actual build phase (the main Ant/Maven build).
+ Use this only for actions that perform some post-processing on the build outputs.
+
+ You can use `$$name$$` to substitute the path to a referenced
+ srclib - see the `srclib` directory for details of this.
+
+ You can use `$$SDK$$` and `$$NDK$$` to substitute the paths to
+ the Android SDK and NDK directories respectively. The following
+ per-build variables are available likewise: `$$VERSION$$`,
+ `$$VERCODE$$` and `$$COMMIT$$`.
+
+ The path to the output APK is available with `$$OUT$$`.
+
+ This runs in [`subdir:`](#build_subdir) if set.
+
 `novcheck: true`<a name="build_novcheck"></a>
 
 : Don't check that the version name and code in the resulting APK are