docs(license): add links to subdir licenses (gnolang#1218)

<!-- please provide a detailed description of the changes made in this
pull request. -->

Presents the various subdirectory licenses within the main license file
(project root).

Closes gnolang/devrel#17

<details><summary>Contributors' checklist...</summary>

- [x] Added new tests, or not needed, or not feasible
- [x] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [x] Updated the official documentation or not needed
- [x] No breaking changes were made, or a `BREAKING CHANGE: xxx` message
was included in the description
- [ ] Added references to related issues and PRs
- [ ] Provided any useful hints for running manual tests
- [ ] Added new benchmarks to [generated
graphs](https://gnoland.github.io/benchmarks), if any. More info
[here](https://github.com/gnolang/gno/blob/master/.benchmarks/README.md).
</details>

---------

Co-authored-by: Morgan <git@howl.moe>

LICENSE.md

@@ -1,4 +1,8 @@
-Each sub-project folder has its own license terms.
+Each project subdirectory has its own license terms.
 
-* gnovm, gno.land - copyleft derived from AGPL3.0
-* tm2 - Apache2.0
+Gno Network Public License 3.0 (copyleft; derivative work, forked from AGPL 3.0)
+- [Gno.land](gno.land/LICENSE.md)
+- [GnoVM](gnovm/LICENSE.md)
+
+Apache 2.0
+- [TM2](tm2/LICENSE.md)