Add initial version of a coding guide

For more in-depth explanations than in the coding standards.

GUIDE.md

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+Ansible Coding Guide
+=======================
+
+## Introduction
+The purpose of this document is to give an in-depth explanation of
+less obvious points of Ansible development, tips and tricks. Most, if
+not all points in this document deserve a mention in the best
+practices/coding standards document, but the explanation in that document is
+usually brief due to its nature and could be hard to understand for
+someone less familiar with Ansible (or Jinja2).
+
+## Lazy evaluation of variables
+This is a surprising, sometimes useful, sometimes not [1], feature of
+Jinja2 template expansions in variables in Ansible. Ansible expands
+the templates in the variable value when the variable is used, not
+when it is defined, i.e. variables are evaluated lazily [2]. So one
+can do this:
+```yaml
+vars:
+  foo: "{{ bar }}"
+tasks:
+  - set_fact:
+      bar: quux
+  - name: use foo
+    somemodule:
+      someparameter: "{{ foo }}"
+```
+
+`foo` is defined before the tasks even start, but can refer to a value
+which gets defined only during the execution of tasks! So one can use
+a subsequently defined value in a previous variable declaration and
+things continue to work.
+
+An example of this feature is
+https://github.com/linux-system-roles/storage/pull/81 : when defining
+`unused_disk_subfact`, the variable `unused_disks` is used. But this
+variable is not available yet. It is set by
+`get_unused_disk.yml`. Still, it works, because `unused_disk_subfact`
+is actually expanded only after the call to `get_unused_disk.yml`, and
+`unused_disks` is already defined here.
+
+Another consequence is that one variable defined in `vars:` can refer
+to another variable defined in the same `vars:` block, like here:
+```yaml
+  vars:
+    too_large_size: '{{ (unused_disk_subfact.sectors|int + 1) *
+                        unused_disk_subfact.sectorsize|int }}'
+    unused_disk_subfact: '{{ ansible_devices[unused_disks[0]] }}'
+```
+At the point when `too_large_size` will be used, `unused_disk_subfact`
+will be already known, and the expansion of `too_large_size` will work
+properly.
+
+Another place where this is used is here:
+https://github.com/linux-system-roles/timesync/blob/9050dd95314406236bc8869eac2307a6fa932edc/vars/main.yml#L1
+Role variables are loaded when the role starts and
+`ansible_facts.packages` is not defined yet (it is not among facts
+collected by default, we collect it during the role execution). But
+this is not a problem when defining `__timesync_chrony_version`,
+because the template does not get expanded at this moment - it gets
+expanded only when `__timesync_chrony_version` is used, and if it is
+after we collect the fact, everything is fine.
+
+(Beware though, `set_facts` expands variables, because it is like a
+module call. So in `set_fact` you can not refer to variables whose
+values contain templates referring to yet undefined variables. In this
+way `set_fact` serves as a checkpoint that expands everything that's
+given to it and is not expanded yet. This can be useful, too bad that
+facts are global and can not be unset.)
+
+## Setting variables on `roles`/`import_role` statement
+Variables set on the `roles` or `import_role` statement appear like
+parameters, but actually they are made accessible to the whole
+playbook run, just like role variables set in `vars/` - therefore can
+influence further role invocations.
+
+Example:
+```
+- hosts: localhost
+  tasks:
+    - name: show var1 before
+      debug:
+        var: var1
+    - name: import role without a role var
+      import_role:
+        name: emptyrole
+      vars:
+        var1: set!
+    - name: show var1 after
+      debug:
+        var: var1
+```
+Equivalent example:
+```yaml
+- hosts: localhost
+  roles:
+    - name: emptyrole
+      vars:
+        var1: set!
+  pre_tasks:
+    - name: show var1 before
+      debug:
+        var: var1
+  tasks:
+    - name: show var1 after
+      debug:
+        var: var1
+```
+`emptyrole` is an empty role created by
+`ansible-galaxy init emptyrole`.
+
+Running both examples show that `var1` is set both in the first
+(before calling the role) and 
+second task (after calling the role). This can be unexpected, as other instances of setting
+`vars:` on a task (including `import_tasks`) keep the variables local
+to the task, so the variable would be unset both before and after the
+task.
+See
+https://github.com/ansible/ansible/issues/66714
+https://github.com/ansible/ansible/issues/55942
+https://github.com/ansible/ansible/issues/50278#issuecomment-450901131
+
+For this reason, either use `include_role`, which does not suffer from
+the problem, or wrap `import_role` in a `block:` and set the variables
+on the `block:`.
+
+
+[1] In a role, you can not pass a variable containing a reference to
+something like `role_name` to another role and use it to reference to
+the calling role, because the variable will only get expanded in the
+called role to something else - the variable represents the currently
+executing role. https://github.com/ansible/ansible/issues/10374
+
+[2] Another well-known language with this style of variable expansion
+is `make`:
+https://www.gnu.org/software/make/manual/html_node/Flavors.html