docs: Add documentation on constraints support

Signed-off-by: David Shah <dave@ds0.me>

docs/constraints.md

@@ -0,0 +1,37 @@
+# Constraints
+
+There are three types of constraints available for end users of nextpnr.
+
+## Architecture-specific IO Cconstraints
+
+Architectures may provide support for their native (or any other) IO constraint format.
+The iCE40 architecture supports PCF constraints thus:
+
+    set_io led[0] 3
+
+and the ECP5 architecture supports a subset of LPF constraints:
+
+    LOCATE COMP "led[0]" SITE "E16";
+    IOBUF PORT "led[0]" IO_TYPE=LVCMOS25;
+
+
+## Absolute Placement Constraints
+
+nextpnr provides generic support for placement constraints by setting the Bel attribute on the cell to the name of
+the Bel you wish it to be placed at. For example:
+
+    (* BEL="X2/Y5/lc0" *)
+
+## Clock Constraints
+
+There are two ways to apply clock constraints in nextpnr. The `--clock {freq}` command line argument is used to
+apply a default frequency (in MHz) to all clocks without a more specific constraint.
+
+The Python API can apply clock constraints to specific named clocks. This is done by passing a Python file
+specifying these constraints to the `--pre-pack` command line argument. Inside the file, constraints are applied by
+calling the function `ctx.addClock` with the name of the clock and its frequency in MHz, for example:
+
+    ctx.addClock("csi_rx_i.dphy_clk", 96)
+    ctx.addClock("video_clk", 24)
+    ctx.addClock("uart_i.sys_clk_i", 12)
+