Developer guide into 0.11 branch (#101)

* Add developer guide

DEVELOPER_GUIDE.md

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+
+# Developer's guide
+
+## Release management
+
+To do a public release you need to...
+
+- Get the git directory ready for the release
+
+- Tag the repo e.g.
+
+```
+    git tag -a v1.2.3 -m ''
+    git push --tags
+```
+
+- Generate the deploy templates, don't add the `v` to the version number.
+
+```
+    templates/generate-all deploy.zip 1.2.3
+```
+
+- Release
+
+  - Go to github, on Code tab, select tags, and find the right version
+  - Select create release
+  - Select the right previous version and generate release notes
+  - At the bottom of the form, find the upload pad, click that and add the
+    deploy.zip created earlier
+
+- Create Python packages.   You need a PyPi token with access to our repos
+
+  - make packages
+  - make pypi-upload
+
+- Create containers.   You need a docker hub token with acccess to our repos
+
+  - make
+  - make push
+
+## Local build
+
+To do a local build, you need to...
+
+- Consider what version you want to build at, and change this in Makefile.
+  It doesn't really matter so long as there isn't a clash with what's in
+  the public repos.  You could stick with the version that's there, or
+  change to 0.0.0 if you're paranoid about pushing something accidentally.
+
+- If you changed the version to generate templates with your version, or
+  changed deployment templates, you need to recreate launch assets to
+  a deploy.zip file:
+
+```
+    templates/generate-all deploy.zip V.V.V
+```
+
+- Build containers
+
+```
+    make
+```
+
+
+- If you changed anything which affects command line stuff (which maybe
+  you could do if you changed schemas), then
+
+```
+    make packages
+```
+
+  That puts Python packages in dist/  You then need to install some or
+  all of those packages.  Typically you only need -base and -cli to
+  an appropriate environment e.g. use Python `venv` to create a virtual
+  environment and install them there.
+
+
+