docs/manual: add section about size graphing

Document the new graph-size target and its possibilities.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/common-usage.txt

@@ -273,6 +273,47 @@ only other format supported is PNG:
 BR2_GRAPH_OUT=png make graph-build
 ----------------
 
+=== Graphing the filesystem size contribution of packages
+
+When your target system grows, it is sometimes useful to understand
+how much each Buildroot package is contributing to the overall root
+filesystem size. To help with such an analysis, Buildroot collects
+data about files installed by each package and using this data,
+generates a graph and CSVs files detailing the size contribution of
+the different packages.
+
+To generate these data after a build, run:
+
+----------------
+make graph-size
+----------------
+
+This will generate:
+
+* +output/graphs/graph-size.pdf+, a pie chart of the contribution of
+  each package to the overall root filesystem size
+
+* +output/graphs/package-size-stats.csv+, a CSV file giving the size
+  contribution of each package to the overall root filesystem size
+
+* +output/graphs/file-size-stats.csv+, a CSV file giving the size
+  contribution of each installed file to the package it belongs, and
+  to the overall filesystem size.
+
+This +size-stats+ target requires the Python Matplotlib library to be
+installed (+python-matplotlib+ on most distributions), and also the
++argpase+ module if you're using a Python version older than 2.7
+(+python-argparse+ on most distributions).
+
+Just like for the duration graph, a +BR2_GRAPH_OUT+ environment is
+supported to adjust the output file format. See xref:graph-depends[]
+for details about this environment variable.
+
+.Note
+The collected filesystem size data is only meaningful after a complete
+clean rebuild. Be sure to run +make clean all+ before using +make
+size-stats+.
+
 include::eclipse-integration.txt[]
 
 include::advanced.txt[]