Add Apache license file

[tdejoigny-ledger]

Description

Add Apache license file

Changes include

• Bugfix (non-breaking change that solves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)
• Tests
• Documentation
• Other (for changes that might not fit in any category)

[lpascal-ledger]

Needs a rebase on master before merging

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4dd29b6) 59.47% compared to head (8c627bc) 59.47%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #469   +/-   ##
=======================================
  Coverage   59.47%   59.47%           
=======================================
  Files          12       12           
  Lines        1683     1683           
=======================================
  Hits         1001     1001           
  Misses        682      682           

Flag	Coverage Δ
unittests	59.47% <ø> (ø)

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