(#20763) [docs] Add license change explanation

* Add license change entry

Signed-off-by: Uilian Ries <uilianries@gmail.com>

* Manage license attribute

Signed-off-by: Uilian Ries <uilianries@gmail.com>

* Simplify

Signed-off-by: Uilian Ries <uilianries@gmail.com>

---------

Signed-off-by: Uilian Ries <uilianries@gmail.com>

docs/adding_packages/conanfile_attributes.md

@@ -57,6 +57,19 @@ The license attribute is a mandatory field which provides the legal information
   * `LicenseRef-` as a prefix, followed by the name of the library. For example:`LicenseRef-libfoo-public-domain`
 * If the library makes no mention of a license and the terms of use - it **shall not be accepted in ConanCenter** , even if the code is publicly available in GitHub or any other platforms.
 
+In case the license changes in a new release, the recipe should update the license attribute accordingly:
+
+```python
+class LibfooConan(ConanFile):
+    license = ("MIT", "BSD-3-Clause") # keep both old and new licenses, so conan inspect can find it
+
+    def configure (self):
+       # change the license according to the version, so conan graph info can show the correct one
+
+       # INFO: Version < 2.0 the license was MIT, but changed to BSD-3-Clause now.
+       self.license = "BSD-3-Clause" if Version(self.version) >= "2.0.0" else "MIT"
+```
+
 ## Order of methods and attributes
 
 Prefer the following order of documented methods in python code (`conanfile.py`, `test_package/conanfile.py`):