Potential fix for code scanning alert no. 63: Workflow does not contain permissions (#68)

Potential fix for
[https://github.com/AlexJSully/Small-Dev-Talk/security/code-scanning/63](https://github.com/AlexJSully/Small-Dev-Talk/security/code-scanning/63)

To fix the problem, add an explicit `permissions` block to the workflow
or the job to limit the permissions of the `GITHUB_TOKEN` to the minimum
necessary. Since this Markdown lint job does not need any write access
to the repository, you should add `permissions: contents: read` as
either a top-level key (applies to all jobs) or at the job level. Since
there is only one job, both approaches work, but top-level is preferable
for simplicity and is the most common practice. Edit
`.github/workflows/markdown-lint.yaml` to insert the following after the
`name:` line and before `on:` (i.e., after line 1).


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Author: 3 people

.github/workflows/markdown-lint.yaml

@@ -1,4 +1,6 @@
 name: Markdown Lint
+permissions:
+    contents: read
 
 on:
     push: