RFR: Add gitignore for build directory

[Martijn Verburg]

Add gitignore for build directory

-------------

Commit messages:
 - Adding .gitignore from a branch
 - Add gitignore for build directory

Changes: https://git.openjdk.java.net/guide/pull/12/files
 Webrev: https://webrevs.openjdk.java.net/guide/12/webrev.00
  Stats: 8 lines in 1 file changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/guide/pull/12.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/12/head:pull/12

PR: https://git.openjdk.java.net/guide/pull/12