Add doc/usage that explains status and functionality grin at Testnet1 (#392)

doc/usage.md

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+# Using grin
+Welcome! grin is so far useable for testing only. Currently on Testnet1.
+Please try it out and help finding bugs.
+
+## Participate on Testnet1
+
+Please participate in testing grin!
+Ask about errors you see, and try retelling the answers in your own words.
+That brings attention to them, so we can make them more understandable for
+users, and improve the [troubleshooting](FAQ.md#troubleshooting) section.
+
+As a user, you can already try to:
+
+- ☐ sync the chain
+  - BUG: Some block(s) have transaction(s) which seem to crash `grin server`
+  - BUG: chain minority forks occur often, and are not quickly abandoned
+  - BUG: `grin server` can jump between chains and fail to converge on the
+      chain with the most accumulated work.
+- ☑ mine, using any of these PoW implementations:
+  - ☑ reference Cuckoo implementation
+  - ☑ cpu_mean_compat mining plugin (typical performance: 1000 graphs/s)
+  - ☑ cpu_mean mining plugin (typical performance: 2000 graphs/s)
+  - ☐ GPU mining not yet functional
+- ☑ view your wallets "outputs" (that *sometimes* equals spendable assets)
+  - BUG: spendable outputs might be hidden or forgotten.
+    WORKAROUND: Backup, then edit wallet.dat manually.
+    A wallet reconstruction command is in development
+    [#295](https://github.com/mimblewimble/grin/issues/295)
+  - BUG: when chain sync fails, coins might be incorrectly marked "Spent".
+    WORKAROUND: Search-replace "Spent" with "Unconfirmed" and resync
+  - BUG: when you activate mining, outputs are created that can be shown as
+    having a value (50.0) and just awaiting confirmation; but if you don't get
+    at least 1 confirmation within 1-5 minutes you can be sure they never will.
+    WORKAROUND: wait until chain can fully sync before you start mining, and
+    then babysit your mining process in case it forks off
+- ☑ send simple transactions (the transaction format is not final)
+  - BUG: after you create a transaction, the outputs it consumes are Locked
+    until the transaction confirms.
+    WORKAROUND: create transactions on the command line, use `-s smallest`
+    and if your recipient fails to claim in a reasonable time, you can always
+    claim it back yourself, loosing only the fee. Or do a full resync...
+
+Legend for the above: `☑ Probably yes now, or ☐ Probably not now.`
+
+## Known problems
+When you run into problems, please see [troubleshooting](FAQ.md#troubleshooting)
+and ask on [gitter chat](https://gitter.im/grin_community/Lobby).
+
+Especially if you're not sure you've found a new bug,
+[please ask on the chat]()
+or on the [maillist](https://launchpad.net/~mimblewimble)
+
+And before you file a new bug, please take a quick look through
+[known bugs](https://github.com/mimblewimble/grin/issues?utf8=%E2%9C%93&q=label%3Abug+)
+and other existing issues.
+
+### Developers, developers, developers!
+
+Please see our list of
+[known requests and bugs](https://github.com/mimblewimble/grin/issues)
+and of course come say hello in the chat or the maillist linked above.