Info about open source back ends that support `if` within P4 actions

[jafingerhut]

I have created this issue to record some details about support for if statements on some open source P4 targets, for reference to the P4 community.

Since creating this issue, I have copied the text that was here to my public p4-guide repository, and added more details to it than were ever recorded directly in this issue's text. Here is a link to that article: https://github.com/jafingerhut/p4-guide/blob/master/docs/if-statements.md

[mihaibudiu]

Why doesn't the current BMv2 back end to p4c support arbitrary if conditions. No one has simultaneously been skilled and motivated with enough time to do it.

There's always something more pressing and easier to do, but if you really want this vote for it and I will take a look. There is always an opportunity cost.