doc/HACKING: Update Module.md for the relay module

In the context of the tor code base, a module is a subsystem that we can

selectively enable or disable, at `configure` time.

Currently, there is only one module:

Currently, tor has these modules:

  - Relay subsystem (relay)

  - Directory Authority subsystem (dirauth)

It is located in its own directory in `src/feature/dirauth/`. To disable it,

one need to pass `--disable-module-dirauth` at configure time. All modules

are currently enabled by default.

dirauth is located in its own directory in `src/feature/dirauth/`.

Relay is located in directories named `src/*/*relay` and `src/*/*dircache`,

which are being progressively refactored and disabled.

To disable a module, pass `--disable-module-{dirauth,relay}` at configure

time. All modules are currently enabled by default.

## Build System ##

   contains a list (white-space separated) of the module in tor. Add yours to

   the list.

2. Use the `AC_ARG_ENABLE([module-dirauth]` template for your new module. We

2. Use the `AC_ARG_ENABLE([module-relay]` template for your new module. We

   use the "disable module" approach instead of enabling them one by one. So,

   by default, tor will build all the modules.

   the C code to conditionally compile things for your module. And the

   `BUILD_MODULE_<name>` is also defined for automake files (e.g: include.am).

3. In the `src/core/include.am` file, locate the `MODULE_DIRAUTH_SOURCES`

3. In the `src/core/include.am` file, locate the `MODULE_RELAY_SOURCES`

   value.  You need to create your own `_SOURCES` variable for your module

   and then conditionally add the it to `LIBTOR_A_SOURCES` if you should

   build the module.

   It is then **very** important to add your SOURCES variable to

   `src_or_libtor_testing_a_SOURCES` so the tests can build it.

4. Do the same for header files, locate `ORHEADERS +=` which always add all

   headers of all modules so the symbol can be found for the module entry

   points.

Finally, your module will automatically be included in the

`TOR_MODULES_ALL_ENABLED` variable which is used to build the unit tests. They

always build everything in order to tests everything.

`TOR_MODULES_ALL_ENABLED` variable which is used to build the unit tests.

They always build everything in order to test everything.

## Coding ##

As mentioned above, a module must be isolated in its own directory (name of

the module) in `src/feature/`.

As mentioned above, a module should be isolated in its own directories,

suffixed with the name of the module, in `src/*/`.

There are couples of "rules" you want to follow: