Improve parser modification and build documentation

Removed .../parser/build.sh as this functionality is an out-of-date version of what is found in the main ant build script "parser" task.

Added a license header and updated the portions of the build-and-modify-parser readme found in src/java/org/apache/velocity/runtime/parser/BUILD_README.txt.

Moved improved src/java/org/apache/velocity/runtime/parser/BUILD_README.txt to build/BUILD_PARSER_README.txt as it is not obvious to go looking for documentation in src/java/org/apache/velocity/runtime/parser when you are modifying "src/parser" and running "build/build.xml"

Replaced previous src/java/org/apache/velocity/runtime/parser/BUILD_README.txt with a simplified version that warns about directly modifying files here and points back to build/BUILD_PARSER_README.txt.