Add guide on how to write modules

doc/src/03020-writing-modules.md

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+# Writing an imag module
+
+So you want to write a module for imag.
+That's nice.
+
+This guide helps you getting started.
+It also can help you understanding how imag modules work, so even if you do
+not want to write a full new module, but extend or alter one, this guide may
+help you.
+
+
+## Data layout
+
+First, you have to think about what data you want to store.
+What functionality do you want to provide and what data that creates.
+
+In this example, we're writing a module that stores numbers. We're writing the
+appropriate library for that as well as a commandline frontend.
+
+
+## libimagnumberstorage
+
+We're writing a `libimagnumberstorage` which provides the core functionality
+of our module: Storing numbers.
+
+That library can then be used by other library authors and by the commandline
+interface implementation.
+
+### Setup
+
+So what do we need to do to write this library:
+
+1. Create a new "lib" crate.
+   Because we're writing a "domain" library, we're doing this in the
+   `lib/domain` subdirectory:
+   `cd lib/domain; cargo new --lib libimagnumberstorage`.
+1. After creating the library, we have to add the new library to the
+   `/Cargo.toml` field and add the missing metadata in the new
+   `/lib/domain/libimagnumberstorage/Cargo.toml` file.
+
+That was the setup part.
+Now we can implement our functionality.
+For that, we need to _extend_ two types from `libimagstore`, so we have our
+first dependency here.
+
+### Dependencies to other libraries
+
+3. Put `libimagstore` as a dependency in the
+   `/lib/domain/libimagnumberstorage/Cargo.toml` file.
+   By using
+   `libimagstore = { version = "0.7.0", path = "../../../lib/core/libimagstore" }`
+   we automatically get all the goodness of Cargo, so that releases
+   automagically work as expected, but when developing locally, the local
+   version of `libimagstore` is used.
+   Of course, the `version` has to be the latest released version.
+4. For error handling, we also need to import `libimagerror`.
+5. For easy header-editing, we import `toml` and `toml-query`.
+6. For error-type creating, we import `error-chain`.
+
+### Interface
+
+7. Then, we have to _extend_ two types:
+    1. `libimagstore::store::Store` has to be extended so we can implement a
+       CRUD interface for our special entries.
+    1. `libimagstore::store::Entry` has to be extended so we can get our
+       stored numbers in a convenient way.
+
+Our interface should roughly look like this:
+
+```
+store.get_stored_number("5") -> Result<FileLockEntry, _>
+store.store_number("5")      -> Result<FileLockEntry, _>
+store.delete_number("5")     -> Result<(), _>
+```
+
+You notice that the `Store` returns `FileLockEntry` objects rather than
+`Entry` objects.
+And that's ok. A `FileLockEntry` is a `Entry`, but ensures that we are the
+only ones editing that entry.
+So, we have to implement our number-storing-interface on `Entry` as well:
+
+```
+entry.get_number() -> Result<usize>
+entry.set_number(usize) -> Result<()>
+```
+
+All those "extensions" are implemented as traits which are then implemented
+for `Store` and `Entry`.
+
+Normally, we create new files for that, as well as for the error types we need:
+
+* `/lib/domain/libimagnumberstorage/src/store.rs`
+* `/lib/domain/libimagnumberstorage/src/entry.rs`
+* `/lib/domain/libimagnumberstorage/src/error.rs`
+
+where `store.rs` contains a trait `NumberStorage` and `entry.rs` contains a
+trait `NumberEntry`.
+`error.rs` contains the invocation of the `error_chain!{}` macro.
+Error types from `libimagstore` and others are linked in.
+