Add flags.md

docs/flags.md

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+# Flags
+
+## Side flags
+
+These are flags, which specify, for which side a [version](schema/version.md), a [file](schema/file.md) or
+a [relation](schema/relation.md) was made.
+
+- `client` This flag specifies, that the version, relation or file can be installed on the client side
+- `server` This flag specifies, that the version, relation or file can be installed on the server side
+
+## Version flags
+
+These are flags, which can be set for [versions](schema/version.md).
+
+- `instance` This flag specifies, that this is a version of an instance addon. Instance addons represent instances of 
+Minecraft itself, while non-instance addons have to be installed into an existing instance of Minecraft.
+
+## Relational flags
+
+These are flags, which describe the relation between the addon and [related addons](schema/relation.md)
+or [files](schema/file.md).
+
+- `required` This flag specifies, that the related addon or file is required for the addon. If the addon gets installed,
+than any relation or file, which has this flag set, also has to be installed.
+- `optional` This flag specifies, that the related addon or file is optional for this addon. If the addon gets installed,
+the user should be able to choose, whether he wants to install the relation or file with this flag, or not. This flag
+can be used in combination with the following additional flags.
+
+### Additional flags
+
+These are flags, which can be used in combination with the `optional` flag.
+
+- `require <addon id>` This flag specifies, that the relation or file requires the addon `<addon id>` to be installed.
+- `companion <addon id>` This flag specifies, that the relation or file requires the addon `<addon id>` to be installed
+and has to be installed, if the addon `<addon id>` is installed. In contrast to `require`, this is a two-way dependency.
+- `exclude <addon id>` This flag specifies, that the relation or file can't be installed together with the addon `<addon id>`. 
+
+With all these flags, `<addon id>` has to be an addon, which is defined as a [relation](schema/relation.md).
+As long as there is just one addon with the id `<addon id>` defined as a relation, the namespace may be omitted
+here, even if it is defined with a namespace in the relation section. Otherwise, it is required, to also
+specify the namespace here.
+
+### Relation specific
+
+These are relational flags, which can only be used for [relations](schema/relation.md).
+
+- `included` This flag specifies, that the related addon is included in this one. This also means, that if some
+addon requires the related addon, it can also be installed with this addon instead. 
+- `incompatible` This flag specifies, that the related addon is incompatible to this one. This means, that they can't
+be installed together in the same instance.
+- `launch` This flag specifies, that the related addon should take care of the Minecraft launch process. It can only
+be used, if both this and the related addon are instance addons.