Update CHANGELOG (v0.6)

CHANGELOG.md

 # REAL changelog
 
+## Version [0.6](https://git.ecdf.ed.ac.uk/pguaglia/real/tags/0.6) (2022-04-05)
+
+#### New features
+
+- **Multi-line input**. Relational Algebra expressions issued interactively at
+  the command prompt can now be broken into multiple lines by pressing the
+  RETURN key. If new lines are added after the first one, the initial prompt
+  changes from `$0 >>> ` to ` N ... ` where `N` is the number (≥1) of the
+  current line. The input is considered complete when terminated by semicolon
+  `;`, at which point the whole expression is fed to the parser. Multi-line
+  input is not available for internal commands, which therefore need not be
+  terminated by semicolon.
+
+- The format of the **CSV output in batch mode** can be controlled with a newly
+  introduced `-csv` option. This takes an argument of the form
+
+  `<header> <separator> <quotes>`
+
+  where:
+  - `<header>` indicates whether the first line of the CSV output is the list of
+    attributes of the answer table; the values `1`, `t`, `T`, `y`, `Y` denote
+    "true" (default) and `0`, `f`, `F`, `n`, `N` denote "false"
+  - `<separator>` is a single character to use as a separator for fields; the
+    default is comma `,`
+  - `<quotes>` is a single character used for quoting each field; the default is
+     double quotation marks `"`
+
+  For example, `-csv "0 | \""` sets the header off and uses the pipe `|` as a
+  delimiter instead of the default comma `,`.
+
+- Interactive suggestions and **auto-completion of internal commands**. Pressing the
+  TAB key while typing a command name (starting with dot `.`) displays a list of
+  possible completions for the command. Repeatedly pressing TAB cycles through
+  the available options, which can also be navigated using the arrow keys. The
+  highlighted option can be selected by pressing RETURN. If only one completion
+  is possible, this is chosen automatically. Completion of options is available
+  for commands that take arguments (e.g., `.help`).
+
+#### Changed functionality
+
+- Relational Algebra expressions issued at the interactive prompt must now be
+  terminated by semicolon `;` in order to be parsed and executed; otherwise,
+  multi-line input is assumed. This is not required in batch mode (as before)
+  for expressions supplied with the `-q` option.
+
+#### Enhancements
+
+- The welcome message (with version, license and copyright information) is now
+  omitted when the app is executed in batch mode (issue #6).
+- Separate in-app help for each internal command (issue #14). The `.help`
+  command now takes the name of a command (without the starting dot `.`) as an
+  optional argument and displays the usage information and description of that
+  command only. For example, `.help tables` will display the help message for
+  the `.tables` command. Without argument, `.help` shows the usage information
+  and descripton of all available commands.
+- Improved formatting of syntax trees, using smoother and more readable lines.
+
+#### Bug fixes
+
+- Prevent application from crashing when a non-conforming input is passed to
+  `.add` in the command prompt (issue #13). An error message is now displayed,
+  along with the usage information for the command.
+
 ## Version [0.5.3](https://git.ecdf.ed.ac.uk/pguaglia/real/tags/0.5.3) (2021-10-21)
 
 #### Bug fixes
@@ -37,7 +100,7 @@
 
 - Renaming is now defined as an operation that only accepts one replacement, in
   place of a list thereof. That possibility is retained as a syntactic shortcut,
-  which is parsed as sequence of nested renamings. For example,
+  which is parsed as a sequence of nested renamings. For example,
   `<R>[A->B,B->C](table)` is short for `<R>[B->C](<R>[A->B](table))`.
 
 #### API improvements