buildman: Add some notes about moving from MAKEALL

For those used to MAKEALL, buildman seems strange. Add some notes to ease
the transition.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/buildman/README

@@ -3,6 +3,8 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
+(Please read 'How to change from MAKEALL' if you are used to that tool)
+
 What is this?
 =============
 
@@ -663,6 +665,96 @@ Other options
 Buildman has various other command line options. Try --help to see them.
 
 
+How to change from MAKEALL
+==========================
+
+Buildman includes most of the features of MAKEALL and is generally faster
+and easier to use. In particular it builds entire branches: if a particular
+commit introduces an error in a particular board, buildman can easily show
+you this, even if a later commit fixes that error.
+
+The reasons to deprecate MAKEALL are:
+- We don't want to maintain two build systems
+- Buildman is typically faster
+- Buildman has a lot more features
+
+But still, many people will be sad to lose MAKEALL. If you are used to
+MAKEALL, here are a few pointers.
+
+First you need to set up your tool chains - see the 'Setting up' section
+for details. Once you have your required toolchain(s) detected then you are
+ready to go.
+
+Buildman works on entire branches, so the normal use is:
+
+   ./tools/buildman/buildman -b <branch_name> <list of things to build>
+
+followed by (afterwards, or perhaps concurrently in another terminal):
+
+   ./tools/buildman/buildman -b <branch_name> -s <list of things to build>
+
+to see the results of the build. Rather than showing you all the output,
+buildman just shows a summary, with red indicating that a commit introduced
+an error and green indicating that a commit fixed an error. Use the -e
+flag to see the full errors.
+
+You don't need to stick around on that branch while buildman is running. It
+checks out its own copy of the source code, so you can change branches,
+add commits, etc. without affecting the build in progress.
+
+The <list of things to build> can include board names, architectures or the
+like. There are no flags to disambiguate since ambiguities are rare. Using
+the examples from MAKEALL:
+
+Examples:
+  - build all Power Architecture boards:
+      MAKEALL -a powerpc
+      MAKEALL --arch powerpc
+      MAKEALL powerpc
+          ** buildman -b <branch> powerpc
+  - build all PowerPC boards manufactured by vendor "esd":
+      MAKEALL -a powerpc -v esd
+          ** buildman -b <branch> esd
+  - build all PowerPC boards manufactured either by "keymile" or "siemens":
+      MAKEALL -a powerpc -v keymile -v siemens
+          ** buildman -b <branch> keymile siemens
+  - build all Freescale boards with MPC83xx CPUs, plus all 4xx boards:
+      MAKEALL -c mpc83xx -v freescale 4xx
+          ** buildman -b <branch> mpc83xx freescale 4xx
+
+Buildman automatically tries to use all the CPUs in your machine. If you
+are building a lot of boards it will use one thread for every CPU core
+it detects in your machine. This is like MAKEALL's BUILD_NBUILDS option.
+You can use the -T flag to change the number of threads. If you are only
+building a few boards, buildman will automatically run make with the -j
+flag to increase the number of concurrent make tasks. It isn't normally
+that helpful to fiddle with this option, but if you use the BUILD_NCPUS
+option in MAKEALL then -j is the equivalent in buildman.
+
+Buildman puts its output in ../<branch_name> by default but you can change
+this with the -o option. Buildman normally does out-of-tree builds: use -i
+to disable that if you really want to. But be careful that once you have
+used -i you pollute buildman's copies of the source tree, and you will need
+to remove the build directory (normally ../<branch_name>) to run buildman
+in normal mode (without -i).
+
+Buildman doesn't keep the output result normally, but use the -k option to
+do this.
+
+Please read 'Theory of Operation' a few times as it will make a lot of
+things clearer.
+
+Some options you might like are:
+
+   -B shows which functions are growing/shrinking in which commit - great
+        for finding code bloat.
+   -S shows image sizes for each commit (just an overall summary)
+   -u shows boards that you haven't built yet
+   --step 0 will build just the upstream commit and the last commit of your
+        branch. This is often a quick sanity check that your branch doesn't
+        break anything. But note this does not check bisectability!
+
+
 TODO
 ====