docs/PARALLEL-TRANSFERS: added

docs/Makefile.am

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  KNOWN_BUGS                                     \
  LICENSE-MIXING.md                              \
  MAIL-ETIQUETTE                                 \
+ PARALLEL-TRANSFERS.md                          \
  README.cmake                                   \
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docs/PARALLEL-TRANSFERS.md

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+# Parallel transfers
+
+curl 7.65.0 introduces support for doing multiple transfers simultaneously; in
+parallel.
+
+## -Z, --parallel
+
+When this command line option is used, curl will perform the transfers given
+to it at the same time. It will do up to `--parallel-max` concurrent
+transfers, with a default value of 50.
+
+## Progress meter
+
+The progress meter that is displayed when doing parallel transfers is
+completely different than the regular one used for each single transfer.
+
+  It shows:
+
+ o percent download (if known, which means *all* transfers need to have a
+   known size)
+ o precent upload (if known, with the same caveat as for download)
+ o total amount of downloaded data
+ o total amount of uploaded data
+ o number of transfers to perform
+ o number of concurrent transfers being transferred right now
+ o number of transfers queued up waiting to start
+ o total time all transfers are expected to take (if sizes are known)
+ o current time the transfers have spent so far
+ o estimated time left (if sizes are known)
+ o current transfer speed (the faster of UL/DL speeds measured over the last
+   few seconds)
+
+Example:
+
+    DL% UL%  Dled  Uled  Xfers  Live   Qd Total     Current  Left    Speed
+    72  --  37.9G     0   101    30    23  0:00:55  0:00:34  0:00:22 2752M
+
+## Behavior differences
+
+Connections are shared fine between different easy handles, but the
+"authentication contexts" are not. So for example doing HTTP Digest auth with
+one handle for a particular transfer and then continue on with another handle
+that reuses the same connection, the second handle can't send the necessary
+Authorization header at once since the context is only kept in the original
+easy handle.
+
+To fix this, the authorization state could be made possible to share with the
+share API as well, as a context per origin + path (realm?) basically.
+
+Visible in test 153, 1412 and more.
+
+## Feedback!
+
+This is early days for parallel transfer support. Keep your eyes open for
+unintended side effects or downright bugs.
+
+Tell us what you think and how you think we could improve this feature!
+