- document custom template definitions (#2051)

Signed-off-by: SSE4 <tomskside@gmail.com>

extending/template_system/command_new.rst

@@ -100,3 +100,38 @@ This is a very simple example for a header only library:
 
         def package_id(self):
             self.info.header_only()
+
+Custom definitions
+------------------
+
+Sometimes it's needed to provide additional variables for the custom templates. For instance, if 
+it's desired to have ``description`` and ``homepage`` to be templated as well:
+
+
+  .. code-block:: text
+
+    # Recipe autogenerated with Conan {{ conan_version }} using `conan new --template` command
+
+    from conans import ConanFile
+
+
+    class {{package_name}}Conan(ConanFile):
+        name = "{{ name }}"
+        version = "{{ version }}"
+        description = "{{ description }}"
+        homepage = "{{ homepage }}"
+        settings = "os", "arch", "compiler", "build_type"
+        exports_sources = "include/*"
+
+        def package(self):
+            self.copy("*.hpp", dst="include")
+            self.copy("LICENSE.txt", dst="licenses")
+
+        def package_id(self):
+            self.info.header_only()
+
+now it's easy to overwrite these values from the command line:
+
+    .. code-block:: bash
+
+        $ conan new mypackage/version --template=header_only -d homepage=https://www.example.com -d description="the best package"

reference/commands/creator/new.rst

@@ -58,6 +58,9 @@ Creates a new package recipe template with a 'conanfile.py' and optionally,
                             excluded
       -ciu CI_UPLOAD_URL, --ci-upload-url CI_UPLOAD_URL
                             Define URL of the repository to upload
+      -d DEFINE, --define DEFINE
+                            Define additional variables to be processed within
+                            template
 
 
 **Examples**:
@@ -93,5 +96,4 @@ Creates a new package recipe template with a 'conanfile.py' and optionally,
       $ conan new mypackage/1.0 --template=myconanfile.py  # Single template file
       $ conan new mypackage/1.0 --template=header_only  # Template directory
 
-
   Refer to the section :ref:`template_command_new` for more information about these templates.