📝 add documentation for integration via vcpkg

appveyor.yml

@@ -83,4 +83,8 @@ test_script:
   # More info: https://github.com/nlohmann/json/pull/1570
   - if "%configuration%"=="Debug" ctest --exclude-regex "test-unicode" -C "%configuration%" -V -j
 
-skip_branch_with_pr: true
+# only build PRs and commits to develop branch
+# (see https://help.appveyor.com/discussions/questions/55079-two-builds-per-commit-to-pull-request)
+branches:
+  only:
+    - develop