doc/REPRODUCIBLE.md: describe to do reproducible builds.

This is a separate commit, as it describes the results (as sha256sum)
of doing a test on the previous commit.  Obviously these values will
have to updated after a rebase!

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/REPRODUCIBLE.md

@@ -0,0 +1,113 @@
+# Rusty's Unreliable Guide to The Terrible World of Reproducible Builds
+
+1. The reproducible build system currently only supports Ubuntu 18.04.1.
+2. It requires manual steps.
+3. The input is a source zipfile, the output is a .tar.xz.
+
+## Step 1: Creating a Build Machine
+
+Download the Ubuntu Desktop ISO image for 18.04.1.  I got it from
+http://old-releases.ubuntu.com/releases/18.04.1/ubuntu-18.04.1-desktop-amd64.iso
+
+The `sha256sum` of this file should be
+`5748706937539418ee5707bd538c4f5eabae485d17aa49fb13ce2c9b70532433`.
+
+Do a standard install, but make sure to *uncheck* 'Download updates
+while installing Ubuntu' in the installer (or simply deprive it of a
+network connection as I do below).  I did the following to install under kvm:
+
+	qemu-img create ubuntu-18.04.01.raw 10G
+	kvm -m 2G -cdrom ~/Downloads/ubuntu-18.04.1-desktop-amd64.iso ubuntu-18.04.01.raw -nic none
+
+You can choose a 'Minimal installation': it shouldn't matter.
+
+Once the installation is over, it'll want to restart.  Then make sure you
+disable updates:
+
+1. Left-click on the bottom left 9-dots menu
+2. Type "update"
+3. Click on the "Software & Up.." box icon.
+4. Click on the "Updates" tab at the top of that app.
+5. Uncheck "Important security updates", "Recommended updates" and
+   "Unsupported updates".  You'll have to re-enter your password.
+6. Hit "Close".
+7. If asked, hit "Reload".
+
+If you didn't have a network connection, you'll want to add one for
+the next steps; for me, this meant powering off the build machine and restarting:
+
+	kvm -m 2G ubuntu-18.04.01.raw -nic user
+
+And then ran `sudo apt-get update` after I'd logged in.
+
+## Step 2: Create the Source Zipfile
+
+Create the source zip that the Build Machine will need, using
+
+	./tools/build-release.sh zipfile
+
+For testing (ie. when you're not on a proper released version), you
+can use --force-version=, --force-mtime= and even --force-unclean.
+
+The will place a file into `release/`, eg. `clightning-v0.7.0rc2.zip`.
+
+### Example
+
+If you are on the git commit v0.7.0rc2 (1dcc4823507df177bf11ca60ab7da988205139b1):
+```
+$ sha256sum release/clightning-v0.7.0rc2.zip 
+3c980858024b8b429333e7ee5a545c499ac6c25d0f1d11bb45fafce00c99ebba  release/clightning-v0.7.0rc2.zip
+```
+
+## Step 3: Put the Zipfile Onto The Build Machine
+
+You can upload it somewhere and download it into the machine, or
+various virtualization solutions or a USB stick for a physical machine.
+
+I simply started a server on my host, like so:
+
+	cd release && python3 -m http.server --bind 127.0.0.1 8888
+
+Inside my KVM build machine I did:
+
+	wget http://10.0.2.2:8888/clightning-v0.7.0rc2.zip
+
+
+## Step 4: Do the Build
+
+1. `unzip clightning-v0.7.0rc2.zip`
+2. `cd clightning-v0.7.0rc2`
+3. `tools/repro-build.sh` (use the same `--force-mtime` if testing).
+   It will download the packages needed to build, check they're identitcal to the
+   versions we expect, install them then build the binaries and create a tar.xz file.
+4. The output will be in that top-level directory.
+
+### Example:
+
+If you built from our example zipfile:
+```
+$ sha256sum clightning-v0.7.0rc2-Ubuntu-18.04.tar.xz
+c9b4d9530b9b41456f460c58e3ffaa779cdc1c11fb9e3eaeea0f364b62de3d96  clightning-v0.7.0rc2-Ubuntu-18.04.tar.xz
+```
+
+
+## Step 5: Get the Built Result Off the Build Machine
+
+Again, there are many ways, but for my KVM settings the simplest was:
+
+On the host:
+
+	nc -l -p 8888 > clightning-v0.7.0rc2-Ubuntu-18.04.tar.xz
+
+On the guest:
+
+    nc -q0 10.0.2.2 8888 < clightning-v0.7.0rc2-Ubuntu-18.04.tar.xz 
+
+
+## Step 5: Tell the World
+
+You can find my example artifacts on https://ozlabs.org/~rusty/clightning-repro
+if you want to see why your build produced a different result from mine.
+
+Happy hacking!
+Rusty.