Add doc for appinfo.json (#256)

docs/appinfo.md

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+# How to modify `build/appinfo.json` file
+
+[appinfo.json](../build/appinfo.json) is a file used to keep some documents up to date, it contains a list of all documents, YouTube videos and script tools.
+
+Keeping this file up to date help to maintain all other important document files.
+
+**When modifying this file, make sure it is still a valid `json` file.**
+
+## Files that relies on `appinfo.json`
+
+- [docs/README.md](README.md)
+- [docs/TemplateList.md](TemplateList.md)
+- [tools/README.md](../tools/README.md)
+
+## File Structure
+
+This file uses `json` language and it contains 4 main sections:
+
+- **[apps](#apps-section)**
+- **[youtube](#youtube-section)**
+- **[docs](#docs-section)**
+- **[tools](#tools-section)**
+
+## APPS section
+
+This section is used to link **Documents**, **Tools** and **Videos** to each app in the Template list. If an app doesn't have any of those items, it doesn't need to be listed in the file.
+
+It can have the following elements:
+
+- `Title`: App Title, exactly as written in Template JSON file **(mandatory)**
+- `DocID`: Document **ID** as listed on [docs section](#docs-section) **(optional)**
+- `ScriptID`: Script **ID** as listed on [tools section](#tools-section). This is usually a script to preintall an app **(optional)** 
+- `ExtraID`: Script **ID** as listed on [tools section](#tools-section). This can be any extra script used to maintain an app. This item can be listed as a single value or as an array (e.g. [3, 5]) **(optional)** 
+- `VideoID`: Video **ID** as listed on [youtube section](#docs-section) **(optional)**
+
+Example 1:
+```json
+{
+    "Title": "App In Template",
+    "DocID": 2,
+    "ScriptID": 7,
+    "VideoID": 9
+}
+```
+
+Example 2:
+```json
+{
+    "Title": "Example App",
+    "DocID": 2,
+    "ScriptID": 7,
+	"ExtraID": [3, 10],
+    "VideoID": 9
+}
+```
+
+## YOUTUBE section
+
+Here all YouTube videos are listed. It contains the following elements:
+
+- `ID`: Episode Number or a sequential **ExtraNN** number when it's a related episode but does not contain any number **(mandatory)**
+- `Title`: Episode Title as listed on YouTube **(mandatory)**
+- `URL`: Episode URL, preferable using the link from the playlist **(mandatory)**
+- `DocID`: If a Episode is link to a helper document, list the **ID** here **(optional)**
+
+Example:
+```json
+{
+    "ID": 7,
+    "Title": "Raspberry Pi Docker Monitoring",
+    "URL": "https://www.youtube.com/watch?v=IoD3vFuep64&list=PL846hFPMqg3jwkxcScD1xw2bKX
+index=8",
+    "DocID": 3
+}
+```
+
+## DOCS section
+
+This section will list all generated doc files with a description. **All items are mandatory**:
+
+- `ID`: Sequential ID number to be used as a link within this document
+- `File`: Contains the exact file name (case sensitive) as used within `docs/` folder
+- `Description`: Will be used as a title on the README page
+
+Example:
+```json
+{
+    "ID": 3,
+    "File": "rpi_docker_monitor.md",
+    "Description": "Install and setup instructions for the RPI Docker Monitor"
+}
+```
+
+## TOOLS section
+
+This section will list all script files included within `tools` folder.
+
+List of items:
+
+- `ID`: Sequential ID number to be used as a link within this document **(mandatory)**
+- `File`: Contains the exact file name (case sensitive) as used within `tools/` folder **(mandatory)**
+- `Type`: Type of script (`Install`, `Preinstall`, `Update` or `Extra`). If not listed will default to `Preinstall` **(optional)**
+- `Exec`: tool used to run the file (normally `bash` or `sudo bash`). If not listed will default to `bash` **(optional)**
+- `Description`: Text describing the use of the script **(mandatory)**
+- `DocID`: Document **ID** when file is referenced on a document **(optional)**
+- `wget`: Shortened URL use to download the script
+
+Example:
+```json
+{
+    "ID": 9,
+    "File": "rpi_docker_monitor.sh",
+    "Type": "Preinstall",
+    "Exec": "bash",
+    "Description": "Preinstall Setup for Docker Monitor",
+    "DocID": 3,
+    "wget": "https://git.io/JPXba"
+}
+```