docs: adds C++ constraint docs

docs/constraints/Cplusplus.md

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+# C++ Constraints
+
+These are the C++ specific constraints applied to the MetaModel before reaching the presets. [Read here to get a more general idea on the overall process](../input-processing.md) of converting a MetaModel to a ConstrainedMetaModel.
+
+## Model Naming
+These are the constraints that is applied to model naming. The `Rule key` is what you can use in the options to overwrite the default behavior. See [constraint customization](./README.md#Customization).
+
+|Rule key|Rule|Resolution|
+|---|---|---|
+|NO_SPECIAL_CHAR|No special characters| Special characters are replaced by their name, for example `!` is replaced with `exclamation`. For C++ `_` are an exception to this rule. |
+|NO_NUMBER_START_CHAR|No numbers as starting characters|Default behavior is pre pending `number_` in front of the first character|
+|NO_EMPTY_VALUE|No empty values|Default behavior is to use `empty` as name. |
+|NO_RESERVED_KEYWORDS|No reserved keywords|C++ has a list of reserved keywords ([see the full list here](../../src/generators/cplusplus/Constants.ts))|
+|NAMING_FORMATTER|Must be formatted equally|Model name is formatted using snake case|
+
+## Property naming
+These are the constraints that is applied to object properties and the naming of them. The `Rule key` is what you can use in the options to overwrite the default behavior. See [constraint customization](./README.md#Customization).
+|Rule key|Rule|Resolution|
+|---|---|---|
+|NO_SPECIAL_CHAR|No special characters| Special characters are replaced by their name, for example `!` is replaced with `exclamation`. For C++ `_` are an exception to this rule. |
+|NO_NUMBER_START_CHAR|No numbers as starting characters|Default behavior is pre pending `number_` in front of the first character|
+|NO_EMPTY_VALUE|No empty values|Default behavior is to use `empty` as name. |
+|NO_RESERVED_KEYWORDS|No reserved keywords|C++ has a list of reserved keywords ([see the full list here](../../src/generators/cplusplus/Constants.ts))|
+|NAMING_FORMATTER|Must be formatted equally|Property naming is formatted using snake case|
+|NO_DUPLICATE_PROPERTIES|No duplicate properties|If any of the above constraints changes the property name, we must make sure that no duplicates exist within the same object. If any is encountered `reserved_` is pre-pended. This is done recursively until no duplicates are found.|
+
+
+## Enum key constraints
+These are the constraints that is applied to enum keys. The `Rule key` is what you can use in the options to overwrite the default behavior. See [constraint customization](./README.md#Customization).
+
+|Rule key|Rule|Resolution|
+|---|---|---|
+|NO_SPECIAL_CHAR|No special characters| Special characters are replaced by their name, for example `!` is replaced with `exclamation`. For C++ `_` are an exception to this rule. |
+|NO_NUMBER_START_CHAR|No numbers as starting characters|Default behavior is pre pending `number_` in front of the first character|
+|NO_EMPTY_VALUE|No empty values|Default behavior is to use `empty` as name. |
+|NO_RESERVED_KEYWORDS|No reserved keywords|C++ has a list of reserved keywords ([see the full list here](../../src/generators/cplusplus/Constants.ts))|
+|NAMING_FORMATTER|Must be formatted equally|Enum key is formatted using snake case|
+|NO_DUPLICATE_KEYS|No duplicate enum keys|If any of the above constraints changes the enum key, we must make sure that no duplicates exist within the same enum. If any is encountered `reserved_` is pre-pended. This is done recursively until no duplicates are found.| 
+
+## Constant
+These are the constraints that are applied to constants. Currently, there are no hooks one can overwrite inside it.