Started to define a basic XSD schema for JetBrains external annotations.

JetBrains.ExternalAnnotations.xsd

<?xml version="1.0" encoding="utf-8"?>
<xs:schema id="JetBrains.ExternalAnnotations" xmlns="" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:msdata="urn:schemas-microsoft-com:xml-msdata">
  <xs:annotation>
    <xs:documentation>
      <![CDATA[
Usage:  You only need to add the following to the assembly element:
        <assembly name="Assembly.Name"
          xsi:noNamespaceSchemaLocation="http://jetbrains.com/schema/JetBrains.ExternalAnnotations.xsd" 
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
]]>
    </xs:documentation>
    <xs:appinfo source="http://www.jetbrains.com/resharper/webhelp/Code_Analysis__External_Annotations.html" />
  </xs:annotation>
  <xs:element name="attribute">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="argument" type="xs:string" minOccurs="0" msdata:Ordinal="1" />
      </xs:sequence>
      <xs:attribute name="ctor" type="attributeCtor" />
    </xs:complexType>
  </xs:element>
  <xs:element name="assembly" msdata:IsDataSet="true">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="member" minOccurs="0" maxOccurs="unbounded">
          <xs:complexType>
            <xs:sequence>
              <xs:element ref="attribute" minOccurs="0" maxOccurs="unbounded" />
              <xs:element name="parameter" minOccurs="0" maxOccurs="unbounded">
                <xs:complexType>
                  <xs:sequence>
                    <xs:element ref="attribute" minOccurs="0" maxOccurs="unbounded" />
                  </xs:sequence>
                  <xs:attribute name="name" type="xs:string" />
                </xs:complexType>
              </xs:element>
            </xs:sequence>
            <xs:attribute name="name" type="xs:string" />
          </xs:complexType>
        </xs:element>
      </xs:sequence>
      <xs:attribute name="name" type="xs:string" />
    </xs:complexType>
  </xs:element>
  <xs:simpleType name="attributeCtor">
    <xs:restriction base="xs:string">
      <xs:enumeration value="M:JetBrains.Annotations.LocalizationRequiredAttribute.#ctor(System.Boolean)" />
      <xs:enumeration value="M:JetBrains.Annotations.StringFormatMethodAttribute.#ctor(System.String)" />
      <xs:enumeration value="M:JetBrains.Annotations.InvokerParameterNameAttribute.#ctor" />
      <xs:enumeration value="M:JetBrains.Annotations.AssertionMethodAttribute.#ctor" />
      <xs:enumeration value="M:JetBrains.Annotations.AssertionConditionAttribute.#ctor(JetBrains.Annotations.AssertionConditionType)" />
      <xs:enumeration value="M:JetBrains.Annotations.TerminatesProgramAttribute.#ctor" />
      <xs:enumeration value="M:JetBrains.Annotations.CanBeNullAttribute.#ctor" />
      <xs:enumeration value="M:JetBrains.Annotations.NotNullAttribute.#ctor" />
      <xs:enumeration value="M:JetBrains.Annotations.CannotApplyEqualityOperatorAttribute.#ctor" />
      <xs:enumeration value="M:JetBrains.Annotations.BaseTypeRequiredAttribute.#ctor(System.Type)" />
      <xs:enumeration value="M:JetBrains.Annotations.UsedImplicitlyAttribute.#ctor" />
      <xs:enumeration value="M:JetBrains.Annotations.UsedImplicitlyAttribute.#ctor(JetBrains.Annotations.ImplicitUseKindFlags,JetBrains.Annotations.ImplicitUseTargetFlags)" />
      <xs:enumeration value="M:JetBrains.Annotations.UsedImplicitlyAttribute.#ctor(JetBrains.Annotations.ImplicitUseKindFlags)" />
      <xs:enumeration value="M:JetBrains.Annotations.UsedImplicitlyAttribute.#ctor(JetBrains.Annotations.ImplicitUseTargetFlags)" />
      <xs:enumeration value="M:JetBrains.Annotations.MeansImplicitUseAttribute.#ctor" />
      <xs:enumeration value="M:JetBrains.Annotations.MeansImplicitUseAttribute.#ctor(JetBrains.Annotations.ImplicitUseKindFlags,JetBrains.Annotations.ImplicitUseTargetFlags)" />
      <xs:enumeration value="M:JetBrains.Annotations.MeansImplicitUseAttribute.#ctor(JetBrains.Annotations.ImplicitUseKindFlags)" />
      <xs:enumeration value="M:JetBrains.Annotations.MeansImplicitUseAttribute.#ctor(JetBrains.Annotations.ImplicitUseTargetFlags)" />
      <xs:enumeration value="M:JetBrains.Annotations.PublicAPIAttribute.#ctor" />
      <xs:enumeration value="M:JetBrains.Annotations.PublicAPIAttribute.#ctor(System.String)" />
      <xs:enumeration value="M:JetBrains.Annotations.InstantHandleAttribute.#ctor" />
      <xs:enumeration value="M:JetBrains.Annotations.PureAttribute.#ctor" />
      <xs:enumeration value="M:JetBrains.Annotations.PathReferenceAttribute.#ctor" />
      <xs:enumeration value="M:JetBrains.Annotations.PathReferenceAttribute.#ctor(System.String)" />
      <xs:enumeration value="M:JetBrains.Annotations.AspMvcModelTypeAttribute.#ctor" />
      <xs:enumeration value="M:JetBrains.Annotations.AspMvcControllerAttribute.#ctor" />
      <xs:enumeration value="M:JetBrains.Annotations.AspMvcMasterAttribute.#ctor" />
      <xs:enumeration value="M:JetBrains.Annotations.AspMvcViewAttribute.#ctor" />
      <xs:enumeration value="M:JetBrains.Annotations.AspMvcAreaAttribute.#ctor" />
      <xs:enumeration value="M:JetBrains.Annotations.AspMvcActionAttribute.#ctor" />
      <xs:enumeration value="M:JetBrains.Annotations.AspMvcTemplateAttribute.#ctor" />
    </xs:restriction>
  </xs:simpleType>
</xs:schema>