Add an IDL definition file

This adds the IDL definition file.
It is also cover in the wiki page:
https://github.com/scylladb/scylla/wiki/Serializer-Deserializer-Code-generation

Signed-off-by: Amnon Heiman <amnon@scylladb.com>

IDL.md

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+#IDL definition
+The schema we use similar to c++ schema.
+Use class or struct similar to the object you need the serializer for.
+Use namespace when applicable.
+
+##keywords
+* class/struct - a class or a struct like C++
+  class/struct can have final or stub marker
+* namespace - has the same C++ meaning
+* enum class - has the same C++ meaning
+* final modifier for class - when a class mark as final it will not contain a size parameter. Note that final class cannot be extended by future version, so use with care
+* stub class - when a class is mark as stub, it means that no code will be generated for this class and it is only there as a documentation.
+* version attributes - mark with [[version id ]] mark that a field is available from a specific version
+* template - A template class definition like C++
+##Syntax
+
+###Namespace
+```
+namespace ns_name { namespace-body }
+```
+* ns_name: either a previously unused identifier, in which case this is original-namespace-definition or the name of a namespace, in which case this is extension-namespace-definition
+* namespace-body: possibly empty sequence of declarations of any kind (including class and struct definitions as well as nested namespaces)
+
+###class/struct
+`
+class-key  class-name final(optional) stub(optional) { member-specification } ;(optional)
+`
+* class-key: one of class or struct.
+* class-name: the name of the class that's being defined. optionally followed by keyword final, optionally followed by keyword stub
+* final: when a class mark as final, it means it can not be extended and there is no need to serialize its size, use with care.
+* stub: when a class is mark as stub, it means no code will generate for it and it is added for documentation only.
+* member-specification: list of access specifiers, and public member accessor see class member below.
+* to be compatible with C++ a class definition can be followed by a semicolon.
+###enum
+`enum-key identifier enum-base { enumerator-list(optional) }`
+* enum-key: only enum class is supported
+* identifier: the name of the enumeration that's being declared.
+* enum-base: colon (:), followed by a type-specifier-seq that names an integral type (see the C++ standard for the full list of all possible integral types).
+* enumerator-list: comma-separated list of enumerator definitions, each of which is either simply an identifier, which becomes the name of the enumerator, or an identifier with an initializer: identifier = integral value.
+Note that though C++ allows constexpr as an initialize value, it makes the documentation less readable, hence is not permitted.
+
+###class member
+`type member-access attributes(optional) default-value(optional);`
+* type: Any valid C++ type, following the C++ notation. note that there should be a serializer for the type, but deceleration order is not mandatory
+* member-access: is the way the member can be access. If the member is public it can be the name itself. if not it could be a getter function that should be followed by braces. Note that getter can (and probably should) be const methods.
+* attributes: Attributes define by square brackets. Currently are use to mark a version in which a specific member was added [ [ version version-number] ] would mark that the specific member was added in the given version number.
+
+###template
+`template < parameter-list > class-declaration`
+* parameter-list - a non-empty comma-separated list of the template parameters. 
+* class-decleration - (See class section) The class name declared become a template name.
+
+##IDL example
+Forward slashes comments are ignored until the end of the line.
+```
+namespace utils {
+// An example of a stub class
+class UUID stub {
+    int64_t most_sig_bits;
+    int64_t least_sig_bits;
+}
+}
+
+namespace gms {
+//an enum example
+enum class application_state:int {STATUS = 0,
+        LOAD,
+        SCHEMA,
+        DC};
+
+// example of final class
+class versioned_value final {
+// getter and setter as public member
+    int version;
+    sstring value;
+}
+
+class heart_beat_state {
+//getter as function
+    int32_t get_generation();
+//default value example
+    int32_t get_heart_beat_version() = 1;
+}
+
+class endpoint_state {
+    heart_beat_state get_heart_beat_state();
+    std::map<application_state, versioned_value> get_application_state_map();
+}
+
+class gossip_digest {
+    inet_address get_endpoint();
+    int32_t get_generation();
+//mark that a field was added on a specific version
+    int32_t get_max_version() [ [version 0.14.2] ];
+}
+
+class gossip_digest_ack {
+    std::vector<gossip_digest> digests();
+    std::map<inet_address, gms::endpoint_state> get_endpoint_state_map();
+}
+}
+```
+