Update parser README regarding inline comments

lib/parser/dockerfile/README.md

@@ -20,6 +20,44 @@ Variable substitutions can be specified in the following formats:
 
 If a variable fails to resolve, it is passed through to the resulting string exactly as it appears in the input.
 
+# Inline comments and backslash
+
+Makisu supports inline comments and backslashes in general, but not if the comment is between multiple lines of one RUN directive separated by backslash. For example, these instructions are supported:
+```
+# Print hello
+RUN echo \
+  hello
+# Print world
+RUN echo \
+  world
+```
+```
+RUN echo \
+  hello # Print hello
+RUN echo \
+  world # Print world
+```
+```
+# Print hello
+RUN echo \
+  hello
+# Print world
+RUN echo \
+  world
+```
+But the following is not supported:
+```
+RUN echo \
+# Print hello
+  hello
+```
+This is because `#!COMMIT` is a special keyword for Makisu, we don't know how to handle it if `#!COMMIT` is placed in the middle of a RUN directive; Besides, backslash is supposed to mean "ignore next character" in bash, so the above example is supposed to be equivalent to:
+```
+RUN echo # Print hello
+  hello
+```
+It doesn't seem reasonable to accept such a Dockerfile.
+
 # Directives
 
 The following directives are not supported: ONBUILD, HEALTHCHECK, and SHELL.