[PATCH v3 2/3] doc: Add section about ccache usage

[Uladzimir Bely <ubely@ilbers.de>]

Signed-off-by: Uladzimir Bely <ubely@ilbers.de>
---
 doc/user_manual.md | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/doc/user_manual.md b/doc/user_manual.md
index 01fe7019..535d456a 100644
--- a/doc/user_manual.md
+++ b/doc/user_manual.md
@@ -18,6 +18,7 @@ Copyright (C) 2016-2019, ilbers GmbH
  - [Add a New Image Type](#add-a-new-image-type)
  - [Add a Custom Application](#add-a-custom-application)
  - [Enabling Cross-compilation](#isar-cross-compilation)
+ - [Using ccache for custom packages](#using-ccache-for-custom-packages)
  - [Create an ISAR SDK root filesystem](#create-an-isar-sdk-root-filesystem)
  - [Create a containerized Isar SDK root filesystem](#create-a-containerized-isar-sdk-root-filesystem)
  - [Creation of local apt repo caching upstream Debian packages](#creation-of-local-apt-repo-caching-upstream-debian-packages)
@@ -912,6 +913,22 @@ package build. To invoke it, just call
 `bitbake mc:${MACHINE}-${DISTRO}:<package_name> -c devshell`.
 
 
+## Using ccache for custom packages
+
+While base system is created from binary Debian repositories, some user
+packages are built from sources. It's possible to reduce build time
+for such packages by enabling ccache.
+
+To enable global ccache functionality, `USE_CCACHE = "1"` can be added
+to `local.conf`. If some package requires ccache to be always disabled,
+`USE_CCACHE = "0"` can be used in the recipe despite global setup.
+
+By default, ccache directory is created inside `TMPDIR`, but it can be
+adjusted by `CCACHE_TOP_DIR` variable in `local.conf`. Ccache directory
+`CCACHE_DIR` default value is `"${CCACHE_TOP_DIR}/${DISTRO}-${DISTRO_ARCH}"`,
+that means caches for different distros and architectures are not overlapped.
+
+
 ## Create an ISAR SDK root filesystem
 
 ### Motivation
-- 
2.20.1