I am dumb and can't spell (#1366)

* rename implementor's guide to implementer's guide

* fix typos in more places

polkadot/roadmap/implementers-guide/src/node/availability/bitfield-signing.md

@@ -0,0 +1,25 @@
+# Bitfield Signing
+
+Validators vote on the availability of a backed candidate by issuing signed bitfields, where each bit corresponds to a single candidate. These bitfields can be used to compactly determine which backed candidates are available or not based on a 2/3+ quorum.
+
+## Protocol
+
+Output:
+
+- BitfieldDistribution::DistributeBitfield: distribute a locally signed bitfield
+- AvailabilityStore::QueryChunk(CandidateHash, validator_index, response_channel)
+
+## Functionality
+
+Upon onset of a new relay-chain head with `StartWork`, launch bitfield signing job for the head. Stop the job on `StopWork`.
+
+## Bitfield Signing Job
+
+Localized to a specific relay-parent `r`
+If not running as a validator, do nothing.
+
+- Determine our validator index `i`, the set of backed candidates pending availability in `r`, and which bit of the bitfield each corresponds to.
+- > TODO: wait T time for availability distribution?
+- Start with an empty bitfield. For each bit in the bitfield, if there is a candidate pending availability, query the [Availability Store](../utility/availability-store.md) for whether we have the availability chunk for our validator index.
+- For all chunks we have, set the corresponding bit in the bitfield.
+- Sign the bitfield and dispatch a `BitfieldDistribution::DistributeBitfield` message.