microVM ID: document user-facing changes

Updated CHANGELOG.md and jailer.md, describing the user-facing changes
introduced by the microVM ID feature. Also reformatted CHANGELOG.md to
max 80 chars per line.

Author: dhrgit

CHANGELOG.md

@@ -6,6 +6,8 @@
 
 - Each Firecracker process has an associated microVM Metadata Store (MMDS). Its
   contents can be configured using the `/mmds` API resource.
+- New firecracker command line argument (--id), allowing the user to specify a
+  microVM ID.
 
 ### Changed
 

README.md

@@ -141,11 +141,12 @@ security.
 Firecracker can be started outside the jail as well. To do this, run:
 
 ``` bash
-./firecracker --api-sock </path/to/unix/socket>
+./firecracker --api-sock </path/to/unix/socket> --id <microvm-id>
 ```
 
 If the api socket is not specified at startup, Firecracker will create
-`/tmp/firecracker.socket`.
+`/tmp/firecracker.socket`. Similarly, the `--id` parameter can be omitted,
+in which case, Firecracker will use `anonymous-instance` as an ID.
 
 ### Configure the MicroVM
 

docs/jailer.md

@@ -76,9 +76,10 @@ After starting, the Jailer goes through the following operations:
 - If `--daemonize` is specified, call `setsid()` and redirect `STDIN`,
   `STDOUT`, and `STDERR` to `/dev/null`.
 - Drop privileges via setting the provided `uid` and `gid`.
-- Exec into `<exec_file_name> --jailed`. The `--jailed` command line argument
-  to the target binary is then interpreted by Firecracker, that realizes it’s
-  running inside a jail, and continues the execution accordingly.
+- Exec into `<exec_file_name> --jailed --id <id>`. The `--jailed` command line
+  argument to the target binary is then interpreted by Firecracker, that
+  realizes it’s running inside a jail, and continues the execution
+  accordingly.
 
 ## Example Run and Notes