Improve LICENSE + copyright information & add CONTRIBUTING file

[Ana06]

• Replace LICENSE text file with the official Apache one: https://www.apache.org/licenses/LICENSE-2.0.txt This changes Copyright (C) 2021 Mandiant, Inc. by the following template language that had been incorrectly replaced: Copyright [yyyy] [name of copyright owner]
• Add CONTRIBUTING file with information about CLA and Google's code of conduct to follow Google conventions/policies. We should add other information to this file in the future to make contributing to the project easier.
• Replace the header from source code files. The old header had the confusing sentence All rights reserved, which
  does not make sense for an open source license. Replace the header by the default Google header that corrects this issue and keep floss consistent with other Google projects. I replaced the header using the following script:

  import os
  import re
  
  OLD_HEADER = b"# Copyright \(C\) (?P<year>\d+).* Mandiant, Inc. All Rights Reserved."
  NEW_HEADER = b"""# Copyright %s Google LLC
  #
  # 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.
  
  """
  
  for dir_path, dir_names, file_names in os.walk("flare-floss"):
      for file_name in file_names:
          try:
              file_path = f"{dir_path}/{file_name}"
              f = open(file_path, "rb+")
              content = f.read()
              m = re.search(OLD_HEADER, content)
              if not m:
                  continue
              print(f"{file_path}: {m.group('year')}")
              content = content.replace(m.group(0), NEW_HEADER % m.group("year"))
              f.seek(0)
              f.write(content)
          except:
              continue

  Some files had the copyright headers inside a """ comment and needed manual changes before applying the script.

Related PRs in other Mandiant repositories: mandiant/capa#2556, mandiant/flare-vm#638, mandiant/VM-Packages#1238, mandiant/capa-rules#980

[williballenthin]

thank you @Ana06 !