add example package

Author: perazz

example_packages/custom_module_dir/README.md

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+# Custom Module Directory Example
+
+This example demonstrates the use of a custom module directory in the `[install]` section of `fpm.toml`.
+
+## Features
+
+- Two simple Fortran modules: `greeting` and `math_utils`
+- Custom module installation directory specified as `custom/modules`
+- Shows how modules can be installed to a different location than headers
+
+## Configuration
+
+In `fpm.toml`:
+
+```toml
+[install]
+library = true
+module-dir = "custom/modules"
+```
+
+This configuration will install compiled `.mod` files to the `custom/modules` directory instead of the default `include` directory.
+
+## Testing
+
+To test this example:
+
+```bash
+cd example_packages/custom_module_dir
+fpm build
+fpm install --prefix /tmp/test_install
+# Check that .mod files are in /tmp/test_install/custom/modules/
+```

example_packages/custom_module_dir/fpm.toml

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+name = "custom-module-dir"
+install.library = true
+install.module-dir = "custom/modules"

example_packages/custom_module_dir/src/greeting.f90

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+module greeting
+    implicit none
+    private
+    public :: say_hello
+
+contains
+
+    subroutine say_hello(name)
+        character(len=*), intent(in) :: name
+        print *, 'Hello, ' // name // '!'
+    end subroutine say_hello
+
+end module greeting

example_packages/custom_module_dir/src/math_utils.f90

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+module math_utils
+    implicit none
+    private
+    public :: add_numbers, multiply_numbers
+
+contains
+
+    function add_numbers(a, b) result(sum)
+        integer, intent(in) :: a, b
+        integer :: sum
+        sum = a + b
+    end function add_numbers
+
+    function multiply_numbers(a, b) result(product)
+        integer, intent(in) :: a, b
+        integer :: product
+        product = a * b
+    end function multiply_numbers
+
+end module math_utils