[kotlin] Clarify license for grammar

pmd-kotlin/src/main/antlr4/net/sourceforge/pmd/lang/kotlin/ast/Kotlin.g4

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: Apache-2.0
 /**
  * Kotlin syntax grammar in ANTLR4 notation
  */

pmd-kotlin/src/main/antlr4/net/sourceforge/pmd/lang/kotlin/ast/KotlinLexer.g4

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: Apache-2.0
 /**
  * Kotlin lexical grammar in ANTLR4 notation
  */

pmd-kotlin/src/main/antlr4/net/sourceforge/pmd/lang/kotlin/ast/KotlinLexer.tokens

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: Apache-2.0
 ShebangLine=1
 DelimitedComment=2
 LineComment=3

pmd-kotlin/src/main/antlr4/net/sourceforge/pmd/lang/kotlin/ast/README.md

@@ -1,8 +1,33 @@
 # Kotlin Grammar
 
-Release: <https://github.com/Kotlin/kotlin-spec/releases/tag/v1.6-rfc%2B0.1>
-Source: <https://github.com/Kotlin/kotlin-spec/tree/v1.6-rfc%2B0.1/grammar/src/main/antlr>
+The grammar files for Kotlin are taken from <https://github.com/Kotlin/kotlin-spec>, released under the
+Apache License, Version 2.0:
 
+```
+Copyright 2000-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+```
+
+The grammar files still use the Apache License, but are slightly modified.
+All other files in this PMD module are licensed under BSD.
+
+## Currently used version
+
+* Release: <https://github.com/Kotlin/kotlin-spec/releases/tag/v1.6-rfc%2B0.1>
+* Source: <https://github.com/Kotlin/kotlin-spec/tree/v1.6-rfc%2B0.1/grammar/src/main/antlr>
+
+### Modifications
 
 Some modifications are made in KotlinParser.g4:
 

pmd-kotlin/src/main/antlr4/net/sourceforge/pmd/lang/kotlin/ast/UnicodeClasses.g4

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: Apache-2.0
 /**
  * Kotlin lexical grammar in ANTLR4 notation (Unicode classes)
  *

pmd-kotlin/src/main/antlr4/net/sourceforge/pmd/lang/kotlin/ast/UnicodeClasses.tokens

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: Apache-2.0
 UNICODE_CLASS_LL=1
 UNICODE_CLASS_LM=2
 UNICODE_CLASS_LO=3