improved contributing

Author: oscarotero

CONTRIBUTING.md

@@ -26,3 +26,23 @@ Before create a pull request, please follow these instructions:
 - Write tests.
 - Run `deno fmt` to fix the code format before commit.
 - Document any change in the `CHANGELOG.md`.
+
+## Local development
+
+To use a local version of Lume in your site for testing purposes, change the
+`lume/` import in the import map to the local folder of Lume. For example:
+
+```diff
+{
+    "imports": {
+-       "lume/": "https://deno.land/x/lume@2.5.0/",
++       "lume/": "../lume/",
+```
+
+Alternatively, use [Lume CLI](https://github.com/lumeland/cli) for a simpler
+process:
+
+- Run `lume local --save` in your local Lume clone to set the current directory
+  as the local Lume path.
+- Run `lume local` in your project folder to update the `lume/` import to the
+  local path.