docs: Introduce BUILD.md

BUILD.md

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+Welcome to Activity Finder
+
+# How to build applications
+
+When you use source code of Activity Finder you need to follow steps of how to compile latest versions of javascript applications, included within.
+
+
+## Internals
+
+Activity Finder contains three applications
+
+- Activity Finder v3 - folder /openy_af_vue_app
+- Activity Finder v4 - folder /openy_af4_vue_app
+- Camp Finder - folder /openy_cf_vue_app
+
+## Compillation
+
+In order to compile any of abobe three apps, follow steps below, please
+
+Go to respective folder
+
+Run in shell
+
+```sh
+npm install
+npm run build
+```
+
+After these are finished - you'd see in dist folders latest versions of compiled applications, which will be automatically used by Acitivity Finder on a site.
+You would need to clear drupal and browser/cdn cache after obtaining new versions of compiled javascript files.