Docs: add backup JSON schema docs to repo

docs/BACKUP_JSON_SCHEMA.md

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+# Backup JSON schema
+
+_SeriesGuide can [export and import your data](http://seriesgui.de/help#backup) using JSON text files. Their schema is documented here._
+
+The actual exported JSON is written without indents and comments to save space and increase performance. The following representation is for documentation purposes only.
+
+When importing, not all but a few required values need to be present. SeriesGuide will in most cases fill in missing values on the next update.
+
+## Shows
+
+```json
+[
+    {
+        "content_rating": "MA",
+        "country": "us", // two letter ISO 3166-1 alpha-2 country code
+        "favorite": true,
+        "first_aired": "2018-02-02T08:00:00Z", // ISO 8601 datetime string
+        "hidden": false,
+        "imdb_id": "tt2261227",
+        "language": "en",
+        "last_watched_ms": 1614593199175,
+        "network": "Netflix",
+        "notify": true,
+        "poster": "/95IsiH4p5937YXQHaOS2W2dWYOG.jpg", // TMDb poster path
+        "rating": 9.5, // 0.0 to 10.0
+        "rating_user": 10, // 0, 1 to 10
+        "rating_votes": 100,
+        "release_time": 300, // Encoded 24 hour local time (hhmm)
+        "release_timezone": "America/New_York", // tz database name (Olson)
+        "release_weekday": 4, // Local release week day (1-7, 0 if daily, -1 if unknown)
+        "runtime": 50, // in minutes
+        "seasons": [
+            {
+                "episodes": [
+                    {
+                        "collected": false,
+                        "episode": 1,
+                        "first_aired": 1517558400000, // ms
+                        "imdb_id": "",
+                        "plays": 1,
+                        "skipped": false,
+                        "title": "Out of the Past",
+                        "tmdb_id": 1401623,
+                        "watched": true
+                    }
+                ],
+                "season": 1,
+                "tmdb_id": "81447"
+            }
+        ],
+        "status": "canceled", // see JsonExportTask.ShowStatusExport
+        "title": "Altered Carbon",
+        "tmdb_id": 68421, // required, or set linked tvdb_id
+        "trakt_id": 122265,
+        "tvdb_id": 332331 // if tmdb_id not set, used to look it up
+    }
+]
+```
+
+## Lists
+
+```json
+[
+    {
+        "items": [
+            {
+                "externalId": "62425", // TMDB ID
+                "list_item_id": "62425-4-firstlist",
+                "tvdb_id": 0, // unused
+                "type": "tmdb-show"
+            },
+            // type episode, season and show are legacy items, 
+            // are only displayed if externalId matches a TVDB ID in SeriesGuide library,
+            // so does not work for new shows added to library
+            {
+                "externalId": "5443955",
+                "list_item_id": "5443955-3-firstlist",
+                "tvdb_id": 0,
+                "type": "episode"
+            },
+            {
+                "externalId": "620558",
+                "list_item_id": "620558-2-firstlist",
+                "tvdb_id": 0,
+                "type": "season"
+            },
+            {
+                "externalId": "253491",
+                "list_item_id": "253491-1-firstlist",
+                "tvdb_id": 0,
+                "type": "show"
+            }
+        ],
+        "list_id": "firstlist",
+        "name": "First list",
+        "order": 2
+    }
+]
+```
+
+## Movies
+
+```json
+[
+    {
+        "imdb_id": "tt1022603",
+        "in_collection": false,
+        "in_watchlist": false,
+        "last_updated_ms": 1619620588554,
+        "overview": "Some text.",
+        "plays": 1,
+        "poster": "/f9mbM0YMLpYemcWx6o2WeiYQLDP.jpg", // TMDB poster path
+        "released_utc_ms": 1256162400000,
+        "runtime_min": 95,
+        "title": "(500) Days of Summer",
+        "tmdb_id": 19913,
+        "watched": true
+    }
+]
+```