license: add extension to LICENSE file

The license file without the extension is shown as "raw" text file. The links do not work as they should. As this file
should point to files with non Apache licenses (the most used case) it should be viewed as markdown.

Author: 0xc0170

docs/reference/contributing/guidelines/license.md

@@ -30,7 +30,7 @@ The full text of the license can be found on the [Apache website](http://www.apa
 
 In order to clearly reflect the Apache 2.0 license, please create two text files:
 
-- A *LICENSE* file with the following text:
+- A *LICENSE.md* file with the following text:
 
    ```
    Unless specifically indicated otherwise in a file, files are licensed under the Apache 2.0 license,
@@ -69,7 +69,7 @@ The full text can be found on [mbed.com](https://www.mbed.com/licenses/PBL-1.0).
 
 In order to clearly reflect the PBL license, please create three text files:
 
-- A *LICENSE* file with:
+- A *LICENSE.md* file with:
 
 <pre>Unless specifically indicated otherwise in a file, files are licensed under the Permissive Binary License,
 as can be found in: LICENSE-permissive-binary-license-1.0.txt</pre>
@@ -96,13 +96,13 @@ See the License for the specific language governing permissions and limitations
 
 If you decide to use a different license for your work, follow the same pattern:
 
-- Create a *LICENSE* file with a description of the license situation, following the pattern described in the sections above.
+- Create a *LICENSE.md* file with a description of the license situation, following the pattern described in the sections above.
 
 - Put the full original license texts in separate documents named *LICENSE-XYZ.txt*, where XYZ is the corresponding [SPDX identifier](http://spdx.org/licenses/) for your license.
 
 - Begin each source header with your copyright line, the SPDX identifier and the standard header for the license that applies to that single file, if it has one. (See [SPDX Specification, Appendix V](https://spdx.org/spdx-specification-21-web-version#h.twlc0ztnng3b).)
 
-- If more than one license applies to the source file, then use an SPDX license expression (see [SPDX Specification, Appendix IV](https://spdx.org/spdx-specification-21-web-version#h.jxpfx0ykyb60)), to reflect the presence of multiple licenses in your *LICENSE* file and in each source file.
+- If more than one license applies to the source file, then use an SPDX license expression (see [SPDX Specification, Appendix IV](https://spdx.org/spdx-specification-21-web-version#h.jxpfx0ykyb60)), to reflect the presence of multiple licenses in your *LICENSE.md* file and in each source file.
 
 ### Contributing to the Mbed OS codebase