CONTRIBUTING: Make the first reference to SPDX a link

Chances are not everyone wanting to contribute has heard of SPDX -- provide
a link to what it is on the first mention

Signed-off-by: Bernhard Rosenkränzer <bernhard.rosenkraenzer.ext@huawei.com>

CONTRIBUTING.md

@@ -24,7 +24,7 @@ OpenHarmony project handles contributions as [merge requests](https://docs.gitla
 
 ## SPDX information and REUSE standard
 
-All projects and files for an hosted project **MUST** be [REUSE](https://reuse.software/) compliant. REUSE requires SPDX information for each file, rules for which are as follows:
+All projects and files for an hosted project **MUST** be [REUSE](https://reuse.software/) compliant. REUSE requires [SPDX](https://spdx.dev/) information for each file, rules for which are as follows:
 
 -   Any new file must have a SPDX header (copyright and license).
 -   For files that don't support headers (for example binaries, patches etc.) an associated `.license` file must be included with the relevant SPDX information.