update doc/en structure

doc/en/content-packs.md

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+# Content-packs
+
+Every content pack must have an ID following requirements:
+- name can consist of Capital letters A-Z, lowercase letters a-z digits 0-9, and underscore '\_' signs. 
+- the first character must not be a digit.
+- name length must be in range \[2, 24\]
+
+Content-pack folder having name same as ID may be created in *res/content*.
+Content-pack folder must contain file **package.json** with following contents:
+
+```json
+{
+	"id": "pack_id",
+	"title": "pack name will be displayed in the content menu",
+	"version": "content-pack version - major.minor",
+	"creator": "content-pack creator",
+	"description": "short description",
+	"dependencies": [
+		"pack",
+		"dependencies"
+	]
+}
+```
+
+Example:
+```json
+{
+    "id": "doors",
+    "title": "DOORS",
+    "creator": "MihailRis",
+    "version": "1.0",
+    "description": "doors test"
+}
+```
+
+Content pack picture should be added as *icon.png* file. Recommended size: 128x128
+
+See *res/content/base* as an example of content pack structure.
+
+