README: Add libboost to list of dependencies

Raspberry Pi IPA, which is enabled by default, requires libboost to
compile.
Specify in the documentation its dependencies and how to disable the
IPA.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Ricardo Ribalda <ricardo@ribalda.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

README.rst

@@ -63,6 +63,12 @@ for the libcamera core: [required]
 for IPA module signing: [required]
         libgnutls28-dev openssl
 
+for the Raspberry Pi IPA: [optional]
+        libboost-dev
+
+        Support for Raspberry Pi can be disabled through the meson
+         'pipelines' option to avoid this dependency.
+
 for device hotplug enumeration: [optional]
 	pkg-config libudev-dev