explain how WHILE works

See #180, release log:

• c5b729e by @yegor256: Merge branch 'master' into 180
• 80d137f by @yegor256: Merge branch 'master' of githu...
• d24055a by @yegor256: #180: readme
• 3e43158 by @yegor256: #180: explained
• 0637969 by @yegor256: #180: fixed
• d96e94d by @yegor256: #180: AtChild
• 5f12514 by @yegor256: #180: Data.ToPhi
• 939eede by @yegor256: #180: Data.ToPhi
• b8e4278 by @yegor256: #180: Data.ToPhi
• 815a2d8 by @yegor256: #180: once.xsl removed
• 5989abd by @yegor256: #180: question removed
• 781ae4f by @yegor256: #180: special symbol for _pare...
• 1664f90 by @yegor256: #180: junit
• 56e67ca by @yegor256: #180: more
• 25e5039 by @yegor256: #180: more
• 94954d2 by @yegor256: #180: more tests
• 23ccc26 by @yegor256: #180: more tests
• dfdc7ab by @yegor256: #180: more tests
• dc9bb61 by @yegor256: #180: junit
• 552ed5e by @yegor256: #180: addSourcesRoot
• and 31 more...

Released by Rultor 1.69.1, see build log